When considering shoulder pads for hockey, there are a few key things to look for. The first is that you want to make sure you find a set of pads with protection appropriate to your level of play. A mite or a once-per-week men's league player probably doesnt need the top model with all the bells and whistles. On the flipside, players playing at a high level like select, juniors, high school or college might need some more protection than a bargain bin pad. So one major key is finding the correct level of protection for you and your game.
Another key is fit and mobility. You want to find the pad that doesn't restrict your movement or does so as minimally as possible. When you try on a set of shoulder pads, you want to make sure you can move around. Try to jump, swing your arms, celly, make a shooting movement, dance – whatever you need to do to make sure you have adequate mobility. You just want to be sure that you have no major restrictions in movement. You also want to make sure that the pad doesn't overlap your elbow pad too much. You don't want the two to interfere with one another, but they should meet so there are no large protection gaps. The shoulder pads border the top of your pants in the same way, close but without interfering with each other.
As far as actual measurements go, the two important ones when fitting shoulder pads are the chest and height. Measuring the chest, you want to take a soft tape measure around the body just under the armpits. Combine that with the height of the player, and you can get a good sense of the size needed.
When you have the pads on there's a few key things to look for. The first is obviously comfort and your ability to move around. Next you want to make sure that your collarbone area is protected well and covered by the pad. You want to make sure the strap around the mid section connect correctly. You want them to be snug. You shouldn't have to stretch and pull hard on the straps to get them to connect. You also don't want them to be too loose that they are not holding the pads against your body.