Comp

When selecting a new bicycle, the correct choice of frame size is a very important safety factor. Most full-sized bicycles come in a range of frame sizes. These sizes usually refer to the distance between the center of the bottom bracket and the top of the frame seat tube.

For safe and comfortable riding there should be a clearance of between 25mm and 50mm beween the crotch of the intended rider and the top tube of the bicycle frame, while the rider straddles the bicycle with both feet flat on the ground.

The ideal clearance will vary between types of bicycles and rider preferences. This makes stradding the frame when off the saddle easier and safer in situations such as in sudden traffic stops.
Women can try a men's style bicycle to determine the correct size for the women's model.

The diagram should help you to make the correct choice.