All BMX bikes are low to the ground. So sizing is done by the top tube length, rather than the more conventional (for other types of bike) seat-to-pedal or inside leg measurement. It's being able to reach the handlebars but not feeling cramped on the bike that matters. The key limiting factor is the length of the top tube (from the saddle to the forks headset). There's no sure-fire way to guess your top-tube length without just trying different bikes with different top-tube lengths. Moreover, different manufacturers vary slightly on these dimensions. For instance, my son is under 4' tall and is comfortable on a Junior. However, a general guide for nomenclature and associated top-tube length for racing BMX is:
Micro (sometimes called Micro-Mini): 15-15.5' (rider under 4' tall, wheels typically 18')
Mini: 15.5-16.5' (rider 4' to 4'6', wheels 20')
Junior: 16-17.5' (rider 4'4' to 4'10' tall)
Expert: 17-18.5' (rider 4'9 to 5'4')
Expert XL: 18-19' (rider 5'2' to 5'8')
Pro: 18.5-19.5' (rider 5'6' to 5'10', wheels typically 24'
Pro XL: 20' and over; riders above 6'2' tall or so may need to have frames especially made up for them.
Some suggest that a 'Midi' or 'Mini XL' size bike exists between Micro and Junior, with the top-tube measurement for Junior, and larger bikes all moving up by one inch, and rider heights moving up by 4-6'.
Micro (sometimes called Micro-Mini): 15-15.5' (rider under 4' tall, wheels typically 18')
Mini: 15.5-16.5' (rider 4' to 4'6', wheels 20')
Junior: 16-17.5' (rider 4'4' to 4'10' tall)
Expert: 17-18.5' (rider 4'9 to 5'4')
Expert XL: 18-19' (rider 5'2' to 5'8')
Pro: 18.5-19.5' (rider 5'6' to 5'10', wheels typically 24'
Pro XL: 20' and over; riders above 6'2' tall or so may need to have frames especially made up for them.
Some suggest that a 'Midi' or 'Mini XL' size bike exists between Micro and Junior, with the top-tube measurement for Junior, and larger bikes all moving up by one inch, and rider heights moving up by 4-6'.
