What size bmx fame is suitable for a 11 year old?

Answer 1

20" or 24" wheel bikes may be suitable. 24" bikes will be better for getting around but are not true BMX's (street trick or dirt race bikes)

True BMX bikes are measured in Top Tube Length- all BMX's hav00e approximately the same standover.

24" BMX bikes are considered "dirt cruisers" with the exception of a handful of street trick bikes made for adults over 6'

Answer 2

Pretty much all BMXes use 20" wheels, which makes all frames very similar in size. Get whatever you can afford and has brakes, in a color scheme that your kid likes.
Do note that a BMX is more like a piece of sports equipment than a "regular" bike. They're really very poorly suited for any kind of distance riding.

Answer 3

The overwhelming majority of BMX bikes use 20" wheels, which makes their frames pretty much the same size too. Not much to choose from basically.