Spinal weapons offer no real benefits if you already have unlocked the next level, and I'm guessing they aren't even really spinal. After all, a spinal 12cm laser (reaching the stats of a 15cm one) isn't different from a 15cm laser, except that you somehow can only mount one. But there doesn't seem any problem having a 12cm spinal laser (equivalent to a 15 cm one) next to a bunch of 15cm lasers. Which unless they are turreted, seem to all point forward too, and probably also run along the length of the ship.I like your idea on oversized weapons, but I don't think it would quite meet the need of the op- which I read as wanting to put a massive gun on his ships. Steve seems to have already come up with something along those lines in his Particle Lance change for 7.2
Something for a proper spinal weapon should work like jump engines. Design a 1000 tons spinal railgun, and it can't be used on ships larger than 10000 tons, for example. Maybe make higher capacitor recharge rate technology allow you to make lighter spinal guns, as less space needs to be dedicated to the capacitor. I'm not exactly sure what stats it should affect. I suppose railguns would have more space to accelerate the rounds and have higher velocity and damage. I don't think the other weapons would benefit from having spinal mounts as much.
To keep the oversized lasers, the spinal box could be replaced with an "Oversize Weapon" box (added to other guns too), allowing you to inefficiently size up a weapon to its next tier, with higher size and power requirements than normal because you're bruteforcing through instead of refining the tech (so that if a 12cm laser is normally 4HS with 4 power requirement, upscaling a 10cm laser would make it 5HS big and need 5 power). And there would still be the reduced sized box, since it only concerns itself with the weapon's capacitor and not the focal size. I don't think that'd clutter or complicate things too much. It'd add another usefulness versus size choice, like there are already a few in the game.
I'm not sure having spinal, oversize and reduced capacitor size boxes would make sense on all weapons. Gauss don't even need power, for example.
Couldn't you get the effect you want simply by introducing bigger spinal weapon mount options?