JW, I might be wrong - but it seems to me that the relation between sizes is linear, while the behaviour of light is log... (arithmetic? Im no mathematician.)

without knowing if I'm right, I'd guess that each time you double the size of the enlargement, in terms of total square inches, you have to double the exposure (EV). BUT - I think the trouble comes in where you begin to have to account for the kind of lens, and the angle of view (projection in this case) in each case... obviously a longer lens will have a narrower angle of view/projection... hm... I'm just out of my depth here... I'd better leave that last thought unfinished... better to get a solid answer.