This is for a road bike.
Stand over it. There should be a little clearance between the top tube and your crotch, like at least 1/2 inch.
Adjust the seat height so that when your foot touches the pedal, your leg is near full extension.
Now ride it. If the top tube is too short for you, you'll be sitting up too straight. If too long, you'll be too stretched out.
Mostly, just ride the bicycle for 30 minutes and pay attention to how comfortable you are and how easily you mount and dismount. Try several out at a bike shop.
Fit is first.
Stand over it. There should be a little clearance between the top tube and your crotch, like at least 1/2 inch.
Adjust the seat height so that when your foot touches the pedal, your leg is near full extension.
Now ride it. If the top tube is too short for you, you'll be sitting up too straight. If too long, you'll be too stretched out.
Mostly, just ride the bicycle for 30 minutes and pay attention to how comfortable you are and how easily you mount and dismount. Try several out at a bike shop.
Fit is first.