In BMX bikes there isn't much to choose from, they're almost all use 20" wheels, which means that there isn't much difference in frame size either. Since they're mostly ridden out of the saddle it isn't much of a problem either.
To determine the size of BMX frame you need for your bike, measure the distance from the center of the bottom bracket to the top of the seat tube. This measurement will help you choose the appropriate frame size for your height and riding style.
It depends on the length of the chainstays on your frame.
All "true" BMX bikes are built for 20" wheels, which makes them very similar in size.
No. A BMX is almost entirely ridden "out of the saddle" (=standing up), so size doesn't mean that much. (most) MTBs are intended for some seated riding, which makes frame size much more important.
Any 20inch forks will fit.
Bmx's don't come in sizes. They are all one size.
It's 1/8" width, but the length is determined by the size of the bike frame, and the size of the sprocket/chainwheel you're using.
A BMX stem is what connects the handlebars and forks to the frame.
get a 14-16 inch frame rest stock and upgrade as you go
A BMX bike size chart provides information on the recommended bike size based on the rider's height. It typically includes frame size, top tube length, and wheel size recommendations to help riders choose the right bike for their height and riding style.
The frame
Unbolt everything from old frame, assemble on new frame, done.