Question is what do you want to do with it as well, are you planning to hit up trails/ramps on it at all?
I'm about 5ft8 and a BMX with a 20.75 toptube is spot on for me but that's for trails riding.
BBs up to 2000 and onwards are pretty much all American/standard then, more recently there have been a couple of new sizes (euro and spanish) becoming common probably due to the weight thing.
Personally I'd be looking for a mid/late 90s Standard Bikes Trailboss or motocross 125....beautiful bikes and very, very well made
1993 Kona Hei Hei, 2013 Mutiny Robbo X-Sekt, 2006 Rocky Mountain Blizzard, 2007 Mongoose Sabrosa 29er, 2016 Transition Scout, 1998 Mk 1 Standard Trailboss , 2016 Genesis Tour de Fer, Player Special - 2015 Ted James custom roadie