What's the largest mushroom cloud possible from a coffee cup/grenade sized nuclear bomb? Assuming the coffee cup is $16$oz = $1$lb = $0.4536$kg or $ \sim 450$mL
I did a quick comparison to the W54 assuming there was a linear ratio (wishful thinking perhaps), and got it to be around 8m high.
Height of mushroom cloud: $1450$ ft
Weight of bomb: $58.1$ lb
Volume of bomb: $5.85$ inches in diameter, $15$ inches long
Source, see Socorro test
Assuming comparable height-to-weight ratio:
Mushroom cloud height of $16$ oz ($1$ lb) bomb = $1450$ft / $58.1$ lb $\times$ $1$lb $\sim 7.6$m
Assuming comparable height-to-volume ratio:
Cloud height of $16$ oz ($454$ mL) bomb = $1450$ft / $(\pi 5.85”^2 \times 15”) \times 454$ mL $\sim  8.3$m
But I'm curious what a more realistic value might be, assuming it was designed for that size and assuming maximum efficiency. Thanks in advance. 
 A: WP says:

Of all the common nuclear fuels, Pu-239 has the smallest critical mass. A spherical untampered critical mass is about 11 kg (24.2 lbs),1 10.2 cm (4") in diameter. Using appropriate triggers, neutron reflectors, implosion geometry and tampers, this critical mass can be reduced by more than twofold. This optimization usually requires a large nuclear development organization supported by a sovereign nation.

I measured my coffee cup, and it is clearly smaller in volume than the volume of a spherical critical mass of plutonium. Using all the other fancy hardware described in the second sentence of the quote would reduce the amount of plutonium needed, but would presumably increase the total size of the bomb.
Therefore I think the answer is that the largest possible explosion from a holy nuclear hand grenade is zero, for any common nuclear fuel.
I believe I heard somewhere that according to rough estimates, substances from the nuclear island of stability might have critical masses as small as a pencil eraser. Luckily for our civilization, there is probably no practical way to make bulk quantities of these atoms.
A: Hirosima was 15kt and had a volume of $1.5m^3$, it produced a mushroom cloud of about $17000m$ 
if things scaled linearly the coffee mug would produce a cloud of $0.0005m$ in height
However I really doubt it would be linear, even on the site that you linked there doesn't seem to be much of a correlation between kt and mushroom cloud height, not to mention there are multiple types of clouds each of which has varying heights.
for more information concerning the hypothetical you could check out:
http://nuclearsecrecy.com/nukemap/
http://upload.wikimedia.org/wikipedia/commons/6/66/Nukecloud.png
