Bike Sizing Guide

1. When you straddle the bicycle frame with your feet flat on the ground, you should be able to clear the top tube (crossbar).
2. If you like to sit upright, go for the biggest frame that still allows top tube clearance.
3. If you prefer more athletic riding, particularly mountain biking over rough terrain, you are safest with maximum clearance, so plump for the smallest possible frame - as long as you can still stretch your legs and the bike isn't so short that it cramps your riding style.
4. Although every manufacturer has their own sizing quirks, a bike's frame size is based on the length of the seat tube (the tube that the seatpost goes into).
5. You can get a rough idea of the size of frame you require if you measure yourinside leg - crotch to floor - then subract 9"/23cm or 10"/25cm for a road bike (or a mountain bike used as a road bike) or 12"/30cm for an athletically-ridden mountain bike.  Hence a rider with a 32"/81cm inside leg who owns a 23"/58cm road bike usually requires a 20"/51cm mountain bike.
6. The development of compact frames with sloping top tubes and long seatposts has further blurred the sizing issue.  I'm unfeasibly tall (6'8"/203cm) so frame-builders would once recommend a minimum road bike frame size of 27"/68.5cm.  I now comfortably ride a 21"/53cm Specialized Expedition.

12" Wheel: 12/14" Inside Leg - 2/4 Years
14" Wheel: 16/20" Inside Leg - 4/6 Years
16" Wheel: 18/22" Inside Leg - 5/8 Years
18" Wheel: 19/24" Inside Leg - 6/9 Years
20" Wheel: 21/26" Inside Leg - 8/11 Years
24" Wheel: 24/28" Inside Leg - 9-13 Years

Adult's Bikes:

14/15" Frame: 25-27" Inside Leg
16/17" Frame: 28/29" Inside Leg
18/19" Frame: 30/31" Inside Leg
20/21" Frame: 32/33" Inside Leg
22/23" Frame: 34/35" Inside Leg and Upwards