Sizing and fit
Unlike goaltender pads, there is no real sizing choice when buying a goalie glove. The size of the glove is determined by the age (size) of the goaltender. Manufacturers usually have sizes based on age -> Youth (5-8), Junior (9-12) , Intermediate (13-16) and Senior (17+) gloves. It is therefore relatively easy to determine which glove is the right one for you. The age ranges for each size of glove are only an indication and will vary depending on the size of each goaltender.
When you put the glove on, it should be large enough so it does not feel too tight. Your fingers should not touch the end of the channels and you should be able to easily tie the wrist strap around your wrist. Ensure this strap is tight enough so the glove will not come off but not as tight as to cut circulation.
You should also ensure the straps for your pinky finger and thumb are in the correct position (not too far so your fingers cannot fit in them) and you are able to tighten them. These straps will help you open and close the glove and ensure it stays in the proper position relative to your hand. Finally, some gloves have an adjustment to tighten the padding around the back of your hand. This should be tight to allow the glove to better follow your hand movements. In the particular glove I use, there are two adjustment straps for the hand.