Optimal size of a windmill for a given windspeed Here is the problem: Assume that you have some constant wind speed. I want to run a windmill but I need to decide how big a windmill I want. The size is characterized by the length of the blades, $r$. There are no engineering constraints (I can build perfect, balanced windmills of any size). However, the mass of the windmill goes as $r^2$ and there is some constant friction coefficient on the axis. Given these conditions, is there an optimal $r$? The goal is to generate as much electricity as possible with the set windspeed.
Okay, that was the physics part. If your answer was that the windmill should be built as large as possible, then what are the common engineering problems that occur as we scale up? In other words, why are real windmills not bigger than they currently are? And why are some smaller while others are bigger?
 A: Okay, that was the physics part. If your answer was that the windmill should be built as large as possible, then what are the common engineering problems that occur as we scale up? 
As large as possible is definitively the correct answer. 
The common engineering problems which comes, is that you don't have a big-enough crane to build it. It's practically only few lifts per windmill, so you simply don't use the crawler cranes like seen in nuclear powerplants. Cause you would need a motorway level road between two windmills, or your crane building is a bigger work than the building of windmill. So you use Mobile cranes. The biggest I know is Liebherr LTM 11200
And it's Width 3 m, Length 20 m, Height 4 m, Weight 108 tn.
+You still need 5 normal trucks for counter weights of 202 tn's
+ another special transport for the T7Y boom, L= 20 m, weight 105 tn's. 
And so you need just a "normal road" with "just" 9 m width to transport your 250 tn and 42 m long crane Quickly in almost one piece. Only 150 tn's of counterweights must be delivered separately. 
Video example about the setting up such a crane
Physics is bitch, I would say. 
In other words, why are real windmills not bigger than they currently are? And why are some smaller while others are bigger?
The oldest one's are smaller. The biggest one are the latest versions. 
