You will want to make sure that you get goalie pants and not player pants. Goalie pants have the extra protection on the inner thigh, and on the front thigh as well as on the hip. Some even have an extra bar of foam and plastic on the outer sides of the pant to help pucks from squeaking in the 6 and 7 holes. 

 

If your pants are too big or too small your mobility will be restricted. Once again you want to feel comfortable while being well protected.