Fitting and Measuring

There are two major dimensions of a bicycle that affect fit: height and length.
The height of a bike is also know as its frame size. This is the length of the seat tube -- that painted part of the frame that runs somewhat vertically from the cranks (pedals) to the saddle (not including the seat post). Frame size is mostly independent from wheel size, however, a large wheel requires the bike to be long enough for your foot not to hit the front tire.

Height is all that many people think about. The rider's leg length dictates both the saddle position and the standover height.
The proper adjustment of the saddle for efficient pedaling is to have a small bend in your leg at the bottom of the pedal stroke. Some pointing of the toe is natural and desirable, but you should not feel that you have to point your toe to keep from straightening your leg completely.
With the saddle in the proper position for pedaling, you will not be able to reach the ground except perhaps with the tip of your toe. If you want to put your feet on the ground while sitting on the saddle, be sure to get a bike small enough to allow you to put the saddle too low for proper pedaling. Realize that you will be putting undue strain on your knees.
Standover height or standover clearance is obviously important. When you mount or dismount the bike, you should stand between the saddle and the handlebars, straddling the frame. For a bike that is going to be ridden only on paved or very smooth dirt surfaces, one inch (1") of clearance from your crotch to the top of the frame is adequate. Most people who ride aggressively off road want at least three inches (3") of clearance. Think of this as emergency exit room.

Length is also an important part of the fit of a bicycle. If the handlebars are too close to the saddle, the bike will handle too quickly and you will not feel secure or in control. If the handlebars are too far away from the saddle, the bike handling will be slow to respond and you will have undue stress on your upper body. Much of this is personal and subjective. There is no absolute, correct length. Designers of bikes understand this and rarely agree among themselves how to proportion the length of a bike relative to its height.
For bikes that come in multiple sizes, you should test ride two adjacent sizes and perhaps different brands, all of which allow you to put the saddle in the proper position relative to the pedals. One should feel more natural than the others.
For bikes that come in only one size per model, realize that different models may target different age groups and may have different lengths. This is especially true of BMX, Freestyle and Dirt Jumping bikes.

I want my child to have plenty of growing room (I don't want to have to buy another bike in six months). We understand this. Of course, the choice is yours. To be safer in case of a panic stop the rider needs to feel capable of jumping off of the saddle and straddling the frame -- without injury. This is not realistic on a bike with zero standover clearance and is obviously impossible on a bike with negative standover clearance.
All-terrain (mountain) bikes are designed for extra standover clearance, therefore they are proportioned long relative to the frame size number. Putting a rider on an all-terrain bike that has minimal standover clearance will result in the length of the bike's being way too long.
Although it spoils any chance of a surprise, to get the maximum growing room and know that you are still getting a usable bike, we recommend test riding.
If you want to keep the bike a surprise, measure the rider's inside leg length -- from the crotch to the floor in regular shoes. This is the easiest measurement to work with when comparing standover clearance and saddle to pedal distance.

Why does your fourteen-year old son want a twenty inch wheel bike? So it is out of the way when doing stunts. Most upper-end freestyle and dirt jumping bikes are made long enough for full-grown riders.