First goalies must be able to skate outside of the net to control the puck for their team 10-20 times a game. Can you imagine putting in a player that led to 10 less possessions?
Second they may take up 50% of the goal compared to 25% of a normal human (given that the person isn't morbidly obese and bed ridden). But they won't be able to move at all. If they stand up chances are they let up most shots (ice level). If they play on the ground then any flip shot is a goal.
Third would be player safety. Given that your goalie will probably never be able to control the puck on shots, guys would just tee off on the guy all game. It will probably be close to impossible to pad the big guy and stay within NHL equipment guidelines (leg pads are restricted in width).