You'd have to do that, piecemeal, for every single character's entire attack moveset (including alternate specials, if the character has them). It would be very easy to do (as you say--just max out the size of the hitbox), but also VERY tedious. There would be well over 500 moves you'd have to change hitboxes for. It would be similar to having to take inventory...of a 500-piece jigsaw puzzle, to make sure there are no duplicate/missing/extra pieces.
But, if someone were to successfully endure that level of nearly insanity-inducing tedium, it would result in THE most broken and crazy-fun Sm4sh yet!