From David Fiedler,
Your Guide to Bicycling.
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For most riders, the first step in getting the right size bike is to stand over the frame with both feet flat on the ground. A properly-sized road bike frame will give an inch or two clearance between the top tube of the frame and your crotch. Not too much, not too little. A mountain bike should have more space - maybe the width of your hand across your fingers.

On women's bikes that don't have the high top tube going between the seat and the handlebars, you can skip this step.