You’re shopping for a new bike and frame size has you stumped. Some bikes have the standard frame design you’re accustomed to. You could simply buy one the same size as your old bike. But the shop also has a selection of non-conventional road bikes with sloping top tubes or other exotic shapes, and these are offered in only 3 or 4 sizes. A bike’s size is more important when it has a conventional diamond frame. With a horizontal top tube, stand-over height is more crucial than with a sloping top tube.