i got nothing on the pot size, but with most plants a safe rule of thumb is that the canopy without training will only be slightly larger then your root mass, just like trees out in your yard, if the canopy is 20ft out from the trunk its safe to say that the roots are close to 20ft out from the trunk as well, so yes larger "leg room" gives you a larger canopy, with the autos that im running now the leaves that are sticking out the furthest are only sticking out a few inches from the outer edge of the buckets they are in, good think to keep in mind. other then that everythings lookin good,
only thing i can think of that might help with more root room in such a small area like your cabinet, would be a square pot that fits the cabinet almost perfectly with a divider in the middle giving you half the grow area per plant assuming you stick with a max of 2 in that space. otherwise youll have some dead space like you do now since round pots of proper size gotta be staggered in opposing corners like you have now. most likely it would have to be a custom square box with divider unless you can find something in a store that nearly fits perfectly, remember more root space = larger plants, this could be bad for you since your space is so small and it could be a blessing if you do some serious bondage or a scrog to make things easier.
just a few thoughts that may help