I don't see extra ammo storage as overpowered or "unlimited" in any way. All it does is give players a choice to sacrifice a slot in exchange for not needing to dock as often or fiddle around with synthesis as much. I for one have no desire to break off combat, run and hide, spend precious materials and time staring at the synthesis menu. This game already has us plodding around the menus enough, thank you; I'd be willing to trade space for something that lets me focus on the ACTUAL gameplay a bit more. As far as module size? There are already easy solutions for that.
1: limit the module size to a class 1 or 2. I agree that a size 8 ammo rack WOULD be overkill, no matter the build or number scaling. I'd never burn a size 8 on ammo anyway, but that's a moot point.
2: limit the ammo supplied based on module size and weapons equipped. Everything gets a percentage more ammo. Maybe like...25/50% for size 1/2? Testing and tweaking needed, obviously.
Of course, I honestly feel like cargo racks should only be needed for precious, fragile cargo that needs to be kept delicately in place anyway and empty slots should just be innate cargo space to begin with...but that's another discussion entirely. I could see ammo racks having a place in ED...provided there's some intelligent playtesting of course.