Fitting your powerlifting/gym belt is choosing the right size.To determine the correct size, measure your waist at the height you would wear the belt and choose a belt that fits that size.
A general thumb rule is measure round your abdomen keeping the measuring tape right at the centre of your belly button in a relaxed position. don't inhale or squeeze and keep a margin for 2-3 inches because size of your belly will change as you gain or loose weight and build muscle.