How do I choose the right size trail bike?

Focus on reach, not seat tube length or traditional sizing (S/M/L). Reach is the most important metric for modern mountain bike fit because it determines your position while standing — your primary position on technical terrain. Typical reach ranges: S (415-430mm), M (435-455mm), L (460-480mm), XL (485-510mm). If you're between sizes, size up for stability at speed or size down for nimble handling. Test ride if possible, as suspension kinematics and geometry feel vary between brands. When in doubt, consult the manufacturer's size chart and consider your riding style — aggressive riders often prefer longer reach.