What size bike is good for a 12 year old?

The boy's height would be more relevant than his age. The basic requirement is that when he straddles the bike he should have an inch or two of clearance between his crotch and the top tube of the frame. But what you really need to consider is what he wants to do with the bike. A BMX is more of a piece of sports equipment than a bike. If you want to take your kid on rides with you a BMX isn't particularly well suited, they're meant to be ridden very energetically out of the saddle during fairly short bursts. Many think of BMX bikes as being suitable kids bikes because their overall size is small, but that's pretty far from the truth. Usually they come with 170-180 mm cranks for instance, and that is the same length as for full-sized adlut road bikes for instance. Same goes for the handlebar. If your kid, when he's in the saddle ends up with the handlebar at about face height it's far too high for him to be able to ride effectively. Anyway it's not like you have a lot to choose from, as 20" bikes have a solid domination over the market.