I'm set that I'm going to build a fixed gear (Flip/flop fixed/free). I haven't selected gear ratio or the like yet..

But, frames? Are there prefered frames, (road, MTB)? I have an old cromoly MTB frame that I could convert, or should I stick to finding a road frame? The fixed gear bike is to help me improve on my roadbike I ride now, so I'm thinking a fixed gear road bike would be preferable. But I"m open to opinions.

Do I select a slightly smaller size than normal?

Would steel be preferable?

Are there certainthings to watch out for?

Etc?