Goal pants are much bulkier and restricting than a standard player pant. With increased padding between the legs and across the hips, movement is reduced while protection is increased. Due to this, the goaltender can wear a player pant until approximately Peewee level hockey, to encourage movement and allow them to gain a better understanding of saves without reduced range-of-motion.
When fitting a goal pant, there is only one major key in terms of fit: the leg length. A perfect fitting pant will sit 1-2 finger widths above the top of the kneecap when the goaltender is in a squat position. If the pant comes onto the kneecap, the pant will be too long and interfere with the goal pads. A pant that is too small will be approximately a hand width above the kneecap, creating an exposure when the goaltender moves into the butterfly position.
Waist size is entirely up to the goaltender. While some goalies may still like the pant to be tight on their waist, it is becoming more common for a pant to be worn loose in order to increase movement and net coverage. Suspenders are a simple fix for a goaltender wanting the best of both worlds.