Bicycle sizes

Question
How do I measure a bicycle for a comfortable and safe ride?

Answer
Anna, Your question has been floating around in the pool and I discovered it!
What kind of bicycle? Road race, mountain, commuter? Bicycles are as different as there are types of cars.
Here are some generic hints.
When looking at a bicycle, straddle it and reaching one hand in front of you and one behind you, grab the top tube and pull it up into your crotch. You need to be able to pick the bike up 1 1/2 to 2 inches off the floor. If the bike is too high and you come off the seat while riding you will hit the frame before your feet hit the ground. Male or Female, that hurts! If the bike is too low the seat post will be too high for safety. With a dropped top tube or a stepthrough frame use an imaginary line from where the seat post joins the frame to where the handlebar post joins the frame.

Put the front wheel in a rack, have someone hold the wheel or ask if you can mount the bike in a trainer, then get on the bike. Pedal backwards and notice your leg extension. With the pedal at its lowest part of its circle, your leg should be only ALMOST straight. Your knee should not be locked. If you have to shift your weight or rock side to side the saddle is too high. If your knee has more than 5 degrees angle you lose peddling efficiency. Once you have found this size the seat post should only be up 4 to 5 inches.
Grab the handlebars. Your hands should be about shoulder width. Closer than that you constrict your breathing. Regardless of the type of bike your weight should not be on your arms, not leaning on the bars. Your elbows should be bent like your knees. Your slightly bent elbows act a shock absorbers.
For any finer adjustments one needs to know the type of bicycle and what kind of riding.