I just want to point out that even if everybody used the same negative format (say, 3 x 2) and didn't crop, adding an even margin all round (which could be 1/4", or 3", or more), or allowing for the strip covered by the frame moulding, will change that proportion of width to height.
So it's actually impossible to 'design' a paper size or a frame size that will suit even the majority of 35mm users, let alone those who crop to other proportions, use multiple negative formats, etc.
When I bought some off-the-shelf frames recently, it was the outside measurement that was standardised (e.g. to 10"x8", A2, A3, etc), and since the moulding width varied between manufacturers, the dimensions and proportions of the visible area were variable amounts smaller than the nominal size.