Would be dx/sx = dy/sy where dx= distance to x and sx = size of x. This also makes the assumption that the two objects are oriented in such a way that the perpendicular plane of the dimension in question for both objects would cross through your perception point if extended infinitely. If these underling assumptions are true, its pretty straightforward. However, if they are not then it is way too much to comprehend. Luckily for astrophysicists most shapes in the universe seem to follow a more circular/ellipsis pattern! I think OP calcs are good enough for our purposes! Bravo