How to choose right size of your bicycle?

You can enjoy our bicycle rides only if your bicycle fit you. You spend less energy, don’t get tired longer, ride faster, smile more only if your bike fits you. And if you bike fits you you can seat on it properly.
Proper seat.
There are two different kids of proper seat: leisure and sport. When you ride for leisure or commuting using your classic dutch bike or some similar bicycle your body position is almost vertical, upright (sometimes exactly vertical if you ride cruiser bike). Your neck don’t strain, you can watch environment, road situation, or chat when you ride. Main body weight falls in this position on the saddle, which for should be wide and comfortable (otherwise you will have back pain ned day after long ride). Handlebar is usually located a few centimeters above the level of the seat.
LEISURE SEAT (dutch classic style)
Sport means higher speed and seat on the bike is optimal for achieving them. Handlebar is located slightly below the saddle level, the body is horizontal, providing better aerodynamics. Cyclist weight more evenly distributed between the arms and legs. A seat for sports more narrow and hard. Rider does not actually seat on the seat. He (or she) is pedaling and weight as it is distributed between the seat, pedals and handlebar.
There are so called Semi-upright position for the best compromise between speed and comfort. This position can be very popular among unprofessional riders, who want to workout when commuting. 
In all cases, the hands should be slightly bent to compensate road bumps.
LEGS (Important!!!) 
In the lowest position of pedals your legs should be almost straight. If you put your heel on the pedal, the leg should be perfectly straight, and if the toe, you get an easy bend at the knee - when you pedal, leg should be stationed on the pedal as shown in the figure. So you envolve to pedalling additional ankle muscles. In this position, the load on the muscles and joints is optimal, that allows you to go faster, avoid unnecessary strain on your knees.
Your leg must be almost straight at the very bottom position.
The most common mistakes
If the bike is too small for you, you have to push up the seat too much, while the steering wheel usually does not have such possibility, as the seat and your body will greatly bent forward, your neck will strain, back tired. If you do not follow the rule of the straight leg (see above), the legs get tired a lot stronger, and you cant ride fast.
Bicycle must not be too small or too big for your height.
Bicycle sizes
Talking about the size of the bike, we usually mention the size of the frame, namely the length of the seat tube, which is measured from the upper edge to the axis of the carriage (pedals). Here we will specify dimensions in centimeters. If this size is larger, all other dimensions of the frame increase proportionally. So for convenience this is usually indicated  this (only one) size. 
In general, the choice of bike for men and women is not much different, except that men ride usually faster and more aggressively, and have shorter legs in proportion to the body. Therefore, usually the size of the frame for a woman to be a little more than men for the same height in both.Note that at the same leg length of different people may differ . Therefore ideal if you have the opportunity to personally try bicycle and feel the size suits you or not.
Children's bicycles sizes
With children's bicycles things are simpler: the right seat is leisure. Bicycles for children usually are choose by height and age, and critical parameter appears not frame size, but wheel diameter. All of the above mentioned rules of proper seat applies to children bicycles with the exception that sometimes parents have to lower down the child seat so that he could feel more confident and easily reach to the ground. It is ok, when the child will get more confident it is better to adjust the seat to reach perfect seat.
Bikes with wheel size below 20" (14-16-18 inches) is usually completed by attached training wheels for those who will only learn to ride .