Actually pedals come in two different sizes 1/2" which are used on older bikes and those with one piece metal crank arms and kids bike. Then there are 9/16" pedals which are most common.
I suggest going to a shop that deals with a lot of BMX bikes. You should probably stick to a platform pedal vs. clipless for now. You can get some decent platform pedals for about $20.00. Prices can go up to $250 for pedals. The difference being mostly with the types of bearings.
A couple tips, a 15mm open end wrench is what you'll need to remove/replace the pedals. Non-drive side pedal is reverse thread, so righty tighty / lefty loosey doesn't apply!
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