For many years my dad was on the ski patrol and worked in avalanche control. At one point in this career, he was the instructor in a weekend class he refers to as "Snow Physics". It was basically a how and why hazardous conditions build up and how they can be spotted early. One of the things he mentioned to me once was that as the temperature drops, the flakes get smaller. The reasons for this were never explained, but I'd have to say it's because the snow is still a bit wet and it sticks together better. My observations have been that when it's warmer, the flakes come down in masses rather than individuals.
I also asked him if it was possible for it to get too cold to snow. He said it was, but it wasn't likely to happen outside of the poles, because the cold will draw all the water out of the clouds on the outer edges of the really cold zone.
I'd actually like some verification on that if anyone knows for sure. Not that I don't trust him, but I'm not quite ready to list him as a source on this.
About photographing flakes: I gave it a shot a few weeks ago. We had a few really cold nights and when that happens, there will be snowflakes that can be easily seen as individuals against a dark background, such as my car. I have a 300mm lens with a macro setting so I was able to find a spot where there were just three in the frame. Then technology struck.
As it turns out, the focal plane of the film and the viewfinder are about 0.75 mm different. (I leaned this by messing up a picture of a jumping spider). Also the camera used the dark background of the car for the exposure settings rather than the flake in the center, so I ended up with a blurry, over exposed white blob, that may well be a streetlight if you didn't know better. I waited three years to catch those conditions too.
BTW, it didn't actually snow that night. It just cold and very humid.
I'm Not Evil.
An evil person would do the things that pop into my head.