As a part of a paper (some school stuff) covering my NXT AutoPilot I went into details of choosing an airframe. I thought that this might be of intrest to you as well.
Here is something to consider while choosing your airframe:
(, where F(i) is drag, P is required power, v is the required speed to keep the airframe aloft, m is the mass of the airframe, A is the wing area and k, rhoo, and C(v) we can assume to be constant.)
From the equation we see that if the weight of our airframe doubles the needed power to keep it aloft gets multiplied by almost three: 2^(3/2))=2,818... Also if the wing area doubles we need only about 3/4 (actually about 0,7) of the power we needed before (assuming the coefficients k and C(v) are constant). This is, of course, based on idealized situations but it's probably something worth keeping in mind while choosing your airframe since the more power you consume the more expensive your set up gets and your flight times get shorter. You might also want to consider the following:
, where 'a' represents the scale (size coefficient) of the airframe (again a highly idealized situation). But if we combine this equation with the first equation we come to the conclusion that the smaller the airframe the better. If we, for example, choose to use a model half the size of some other airframe we are going to consume only about 1/10th of the power
we would use with the bigger model (if we assume the wing area to be roughly 1/4th of the bigger airframe and the shape of the both airframes to be identical).
You should note, however, that these calculations only take the power needed to stay aloft into consideration and if you carry a payload the weight of which doesn't vary with the size of the airframe you will probably have to resort to bigger airframes.