Try to simplify the body in to smaller 3d shapes..... look at some references online.......
make a straight line going from the shoulder to the wrist. then, make a line perpendicular to the one you just made, touching its middle. however long the second line is, if you put the character's elbow at the end of it, it shouldn't look off.
example of what you need to do, poorly done in MS paint with a trackpad because I don't have my tablet with me:
