CHALLENGER

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 25 mm and 50 mm beween the crotch of the intended rider and the top tube (horizontal tube below (C), extending from seat post quick release to steering) 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 riders preferences. This makes stradding the frame when off the saddle easier and safer in situations such as in sudden traffic stops.
To assist the wohmen in choosing the correct size, a men's style bicycle could be used for reference..

The diagram should help in mainge the right choice.