Being big does not make a power forward. At all. Stamkos is 6 foot 1, or average in the NHL. Brandon Prust is 1 inch taller than Callahan. Does this mean that we should keep all players of height above 6 feet? .
This is actually a legitimate question. Maybe. It depends of team need for speed, cause tall guys are slower, except elite, while size gives you physical upperhand. Ideally you want to have enough depth to allow coach more freedom.
Callahan is not a role player. Role players are not out there when the team is trailing late by a goal. Role players do not score 29 goals.
Role players motivate by example. It doesn't matter whether they fight, score, hit or just show energy. They lead. Leadership is an ability to make other people to do things that people wouldn't incline to do. Hockey or not. If Messier wasn't a HOF Power Forward, he would a terrific Role Player.
You can call Callahan an elite role player. Still a role player. So is Prust. Rangers didn't think he imay become an elite one. They could be wrong. So could be you.
Anyone can be a playmaker. MSL is a RW, but is probably one of the best playmakers in the game. Stamkos is a center, but will never be mistaken for a playmaker
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This is irrelevant to Prust, but playmakers are the most developped hockey players there are. Anyone can learn to pass well. You have to have an incredible talent to play so called "full deck of cards" to be in such role. Only few show that ant young age. MSLhad, like the rest, to age in order to become one, he was just a sniper when he was as young as Stamkos. I think Stepan is as close as it gets to it, but lack of conststency downs even established ones, like Richards currently. This is because it is requires total concentration every shift. If you have gotten a playmaker that unable play C due to defense inability, size or whatever, you use him on the wing, or as point on PP any position because they are rarities.