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.
The HUGE majority of BMX bikes all use the same 20-inch wheels, which makes the frames very similar in size.
20" - 20.75" should be about right, a 16 or 18" BMX would probably be too small, and besides they will keep growing!
All real MBXes have the same wheel size (20"), so frames tends to be very similar in size too. As bmx bikes mainly are ridden standing up frame size doesn't matter much.
All "true" BMX bikes are built for 20" wheels, which makes them very similar in size.
It depends on the length of the chainstays on your frame.
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
The frame
Unbolt everything from old frame, assemble on new frame, done.
BMX bikes pretty much all have 20"wheels, which makes all frames very similar in size. On top of that BMX is almost entirely ridden standing up, so bike size doesn't matter as much as it does for road riding for instance. I agree, but get at least a 20.5" frame.
In BMX bikes there isn't much to choose from, they almost all use 20" wheels, which means that there isn't much difference in frame size. Since they're mostly ridden out of the saddle it isn't much of a problem either.