We say 'modern' because race bikes have changed since the previous decades. The modern race bike geometry starts at circa 2006-ish depending on the brand.
When most people see sizing, they think of the wheel size. This is how bikes are sold at department stores. BMX race bike sizing differs from road or mountain bike sizing. BMX sizing is done with the top tube (TT). The top tube is the uppermost tube of the frame. To measure the top tube, measure from the center of the seat tube to the center of the head tube. The geometry and feel of the bike is actually determined by the downtube, head tube, bottom bracket, chainstays, and seat tube - basically everything else except the top tube! But the top tube is the easiest measurement to quickly size someone.
BMX race bikes come in these general sizes: Micro, Mini, Junior, Expert, Expert XL, Pro, Pro XL, and Pro XXL. It is important to size a race bike correctly. Too long and it handles like a dump truck. Too short and it can be hard to ride. Ultimately, racing a bike you are most comfortable on is the best advice we can give. The 20” wheel size race bikes are called ‘class’ bikes; 24” wheel size race bikes are ‘cruisers.’ Cruiser sizing for adults is more or less one size, and kid cruiser sizes are mini, junior, and expert. The kid cruisers have become niche and are not as common as they once were.
Note that kid sizes (micro - expert) typically have 1" headsets and ISO 451 tubes and tires. The adult sizes (ex xl - pro xxl) use 1-1/8" or tapered headsets and ISO 406 diameter wheels.
Try out the rental bikes at your local track. Riders will likely have to size up every two seasons until they hit the pro size frames.
Generalized BMX race bike sizing:
Micro: 15" to 16.5” TT. This typically has an 18” wheel (a few may have 20”) and is for ages 3-5.
Mini: 17" to 17.5” TT. Often the first pedal bike for most riders. Ages 5 to 7 and 4'1'' to 4'5''
Junior: 17.25" to 18.75” TT. From about ages 7 to 10 and 4'4'' to 4'9''
Expert: 19" to 19.75” TT. Ages 9-11 and 4'6'' to 4'10''.
Expert XL: 19.5" to 20” TT. This is an in-between size. Many riders will skip this size or be on it for a short time. Height typically 4'10'' to 5'2''
Pro: 20" to 20.5” TT. Ages 12 and up, usually around 5'0" to 5'7" in height.
Pro XL: 20.75" to 21.25” TT. Ages 12 and up, height from 5’7” to 6’0”. Most adult riders use this size.
Pro XXL: 21.25" to 22” TT. Ages 12 and up, 6’0” and taller riders. Riders under 6’ might like the XXL because it is very stable for them but are harder to turn.
