This is a very tricky issue.When we come to a bicycle dealer, he usually measures the inside leg length and calculates your frame size based on this. Nothing is less true, because there is more to it than this. We often conclude that a frame is too big or too small, or that you are like on a mountain bike because the headset is much too long.How did that happen?Well, the trunk and arm length determine your frame size even more.Nowadays, most bicycles are sloping. This means that the seat post is much longer and that we can have a tolerance of more than 20 cm. (if we buy a very long seat post) That has the advantage that we should not really focus on frame height.The top tube length is fixed and only with stem and reach of the handlebars you can vary in length. Length stems are from 60mm to 140 mm. Reach handlebar, 68mm to 115 mm so it means that the variable length is 12.7 cm. Let's not lose sight of one thing, the eye also wants something. A stem from 60mm to 80mm and that of 140mm does not look nice.Then we still have the length of the ball head, which is also not variable. Some producers have bicycles with a rather high head tube and others have a lower head tube.Why is that so important?The sizes of the length of the top tube and height of the headset is a matter of aerodynamically ideal sitting. This is certainly applicable for competitive and cyclo riders.Not only a short ball head is important for an aerodynamic sitting position. There are people who have long legs, a short upper body and long arms. That person should look for a smaller frame with a short ball head, compared to a measurement where only the inner leg length is considered.There are also people with long legs, a long trunk and short arms. They have to look for a frame with a longer head tube.What is our conclusion?Before buying a new bicycle: Have a professional bike fit.