I'm developing an XNA Game. Basically, I have a first person camera. What I want to do is get the position of an avatar's head. The reason I want to do that is because I want the camera to be on the avatar's head thus, giving it a real life view (you can see the hands and that type of stuff.
Usually you just say "my character is X meters tall" and put the camera there. For the player's hands you have a custom model that looks good from first person that's separate from what other people see in the world (assuming you can see other avatars).