Renyolds number. (sp)

Ok that's what you are "missing" Basically, you scale a plane down, but the air molecules remain the same size.

That aside, in a perfectly steady state wind, all planes handle it just fine as they don't even know the wind is there. So if you are in a flat area and high enough off the ground, you can fly anything in any wind. You might not be able to fly it back to the field, it might be going backwards out of sight at 30mph, but all the plane knows about is airspeed, it doesn't know about the ground.

However, in the real world we have turbulance. Edies, up and down drafts, etc. Some of these are smaller than others. If you have a small plane, it is more bothered by smaller bits of turbulance than a larger plane would be. For example, a Cessna 152 has to worry more about the wake turbulance of a 747 than another 747 would. The size of the turbulance compared to the size of the plane does matter, so with models and the small bits of turbulance we run in to when low and slow, a bigger plane is less bothered.