Feathers and Human Flight In the case of Angels where they're supposed to have wings full of feathers and can fly. If we created a flight suit for humans made out of feathers, how big would the feathers have to be? 
If the feathers got scaled up would it make it impossible for flight?
If you wanted to flap in order to gain lift, how much weight would be on each arm due to the feathers and how much would you have to flap to gain thrust?
I don't know whether this is the correct forum for this question - it's sort of half biology half physics. To be honest I'm more interested in the maths and physics behind this question.
 A: Short:
Given enough assumptions to make the question answerable.  
Feather size:
Probably 300 to 500 mm for a small percentage of the feathers
and closer to 50% of that for the majority
Power and force:
Without a complex analysis of flapping flight (with takeoff mode, soaring versus "hovering" capabilities and more) a definitive answer would be difficult, but given the assumptions below, a rough scaling of power with mass seems appropriate, wing loadings (flight mass per area) can be similar and the use of available stronger-than-original non biological materials should allow an approximately linear scaling of flight-hardware mass with total lifted mass. Power levels required for takeoff appear to be well in excess of those achievable from available human biological sources - something confirmed by all human experience to date.
However, the example of birds such as the Great Albatross and the Wandering Albatross indicate that both power and energy levels required for very long range soaring mode flight should be achievable by human biological power plants. Learning how to do this at all, let alone while sleeping, is yet to be mastered. 
Because - read on ...

Longer:
You do not specify whether the feathered flight suit was to be man powered or machine powered - but indications from other areas of aeronautics (and several thousand years of trying) are that transition to flight or maintenance in still air level flight requires more power than the human engine system can provide. So, I'll assume a non-human power source and concentrate on the feathering.
The question has too many variables that affect the answer for the size of the feathers to be a useful indicator of flight ability. Feather size can vary somewhat independently of wing size, and beat rate and stroke also are vitally important. Also of significance to flight is the ability for a given mass and size of animal to provide the energy and power levels needed for flight, and the affect of size on structural strength. 
As a guide to what nature has provided in large bird design in recent times, consider the relatively recently extinct (~700 years) NZ Eagle and currently still out & about Great Albatross and Andean Condor.  
The (believed) heaviest known bird capable of flight to have lived in the last 1000 years was the Haast Eagle - thought to have become extinct in about 1400 AD due to the extinction of its most common food source, the NZ Moa. 
Estimates vary but the Haast Eagle is believed to have weighed in the 15 to 20 kg range with a maximum wingspan in excess of 3 metres.  So body mass is somewhat above the current heaviest flying birds and wingspan somewhat below.
The currently extant Great Albatross has wing spans of up to about 3.5 metres but a somewhat lower body mass.
The Andean Condor is almost as heavy at up to about 15 kg and with a wingspan of up to around 3 metres.
For a very rough first estimate I'll use the following assumptions:


*

*Wind stroke and area of existing large flying birds (and Haast Eagles) are approximately optimum for the load. 

*Modern materials will allow flying related swept area per mass to be as good as or better than nature has achieved in these large birds. 


*

*This violates the usual cube-squared law that usually requires scaled up versions of a given creature to be substantially more sturdy and thick boned, and scaled down versions to be much more "delicate". This is because volume and more oir less mass changes with dimension cubed but area such as bone cross section to change with dimension squared. So a linearly scaled up ant of elephant size would break apart   under its own weight and muscle forces and a linearly scaled down elephant of any size would be vastly too massive for its energy and power capabilities. 


*Feather size scales at most linearly with dimension squared - ie a Haast Eagle scaled up to man mass size would have feathers in the same proportion to size as on the original, or smaller. 

*Wing design will be "something like" the best examples available in nature. While many people probably thing that they could greatly improve on current designs, the experience base is very limited compared to the competition and so far design successes are few. 
So:  Very (very very) roughly, if area is scaled in proportion to mass and beat rate and stroke are the same as percentage of body length, then a "bird" that is scaled up by a factor of 3 length, so having about the same wing area, will be able to deliver about 9 x the thrusts and require about 9 x the power to do so. A biological creation, which relied on similar strength materials and similar fabrication methods, would be overstressed and destroy itself but the assumption about high tech materials allows this with roughly linear mass increase per power. 
So - we have a 9 metre wing span creation able to lift a gross lift off mass of
about 9 x (12 to 20) kg 
~~~~= = 108 - 180 kg.
Which should be enough.
Soaring power requirements:
All human experience to date suggests that the very best designs and materials allow human powered flight only by those of extreme fitness and strength eg the Gossamer Condor (the first man powered aircraft to fly a 1 mile figure of eight course) and the Gossamer Albatross which crossed the English Channel.
All such designs use non-flapping flight with "conventional" aerofoils and propellor propulsion.
However, an examination of the performance of eg the [Wandering Albatross](
https://en.wikipedia.org/wiki/Wandering_albatross#Description) with a range of around 10,000 km and a glide ratio in excess of 20:1 suggests that "man powered" long distance soaring may in fact be achievable. The performance of eg Yves Rossy and his wingsuits to date, while exceedingly impressive,  suggests that "we may have a way to go. (The 'Rossy'  wingsuit  typically has a wingspan of around 2.5 metres or about 30% of what is suggested by the above assumprions as needed for soaring flight.
However, there is a class of 'gliders' that comes close to Albatross specifications. Foot launchables and the bottom end of ultra-light gliders. 
'Goat' short video  - not foot launchable but about 1 foot down-slope takeoff run into breeze. 
Current or recently retired heavy lifters:
Wikipedia - Haast's Eagle - females <= 16.5 kg
NZ birds of prey - Haast's Eagle <= 17.8 kg, almost 3 metre wingspan. 
Wikipedia- - Andean Condor <= 15 kg, <= 3.1 metre wingspan.
Wikipedia - Great Albatross "over 11 kg", wingspan <=  3.5 metres 

Feather length to wing size.
Haast Eagle feathers are as rare as hens teeth.
Images or dimensions of Albatross wings seem hard to come by.
This image of an Albatross wing may seem exaggerated in length to width, but a look at photos of Albatross in flight show that the wings are typically about as long and thin as this indicates. 
Image from here - 
Wikipedia - Albatross wing shape
Albatross in flight examples:

From here - Albatross wing aspect ratio example
And another from here

It can be seen that the longest feathers are in the order of 15% of wing length.
It may be felt that the Albatross gliding biased orientation makes this wing shape a poor one for comparison and that a more lift biased bird - eg a Hummingbird, would be a better candidate for comparison. However, the 3 large birds here have similar  aspect ratios and are proven heavy lifters. It is by no means certain that a Hummingbird design would scale to a 100 + kg lift off mass. However, assumptions may be adjusted.
On this basis lingest feathers are liable to be wingspan/2 x 15% say or ~~=
9m/2 x 15% = 675 mm    
However, in the high aspect wings fearther size varies widely and mosts are much shorter than the maximum. Here is an image of the range of feathers from  a high aspect ratio wing as used by gulls, Albatross and Petrels. It can be seen that anything like maximum feather size occurs only for a small proportion of feather styles (as can also be seen in the whole wing diagram above.) While it's not certain that a feathered flapping wing design could improve on creation, odds are that a few shortcuts in the extreme upper end of the feather range would allow maximum feather length to be reduced and something in the 300-500mm range seems likely with most substantially less than that


Related:
Design of a competition sailplane
Aspect ratios and wing loading for some birds:

From above site.
https://en.wikipedia.org/wiki/Poukai 
http://www.wingspan.co.nz/extinct_birds_of_prey_new_zealand_haasts_eagle.html
http://nzbirdsonline.org.nz/species/haasts-eagle
