The relationship between the motor and platter are the same as any pair of gears. If you add a link to the chain on your bicycle will the speed (gear ratio) change? Of course not. Neither will the speed of your turntable change if you make the belt 1/2" shorter or longer. The speed is determined by the diameter of the motor pulley, the speed of the motor, and the diameter of the platter rim that the belt fits on.

Most turntables have two speed adjustments for the motor underneath the bottom panel. In most cases you will need to take the bottom off to get to them. Some units have access holes on the bottom. This is how you make small speed adjustments, not by changing the turntable belt size.

There are extremes to this. If the belt is way to short it will put too much tension on the motor and will usually cause the motor to run fast. I know that seems backwards but is indeed the case.