Proper way of marking dimensions of a drawing in a paper
In my opinion it depends on why the figure is part of the paper.  Say you wont to emphasize a particular set-up , for instance in experimental section or whatever section dealing with experimental details (size and shape ) because necessary for the discussion, then I would explicitly quote the numbers. 

If you are to discuss relation which are general then use letters

If a wish a particular triangle I would cite three numbers. If I discuss Pytagorean theorem is fine and more useful to draw A, B e C.