Ok, so here's what i ended up doing.
I made flagship hulls, edited versions of all 3 of the huge hulls, a little better than regular huge hulls, about 12 times more expensive. 800 space on all of them.
Made two modules out of the most basic support module. (They do all minimally increase logistical range too. A nice secondary perk.)
One is a general hull mod that scales with ship size, it costs as much as a tiny hull, which is 25, and on a tiny hull it adds about 21 space (which is a little more than stock capacity on a tiny hull). Am probably going to increase the cost to about 50, but it will effectively double the cost of that ship and a little more than double the capacity.
No way I know of to control size specific extension modules, other than to make 4 other hull capacity modules and tailor them to each specific size of hull. I figure i'll just play with this the way it is now and see what the AI does with them.
The 2nd module costs 5, adds less than the regular hull mod does, but obviously costs less to do. The mathematical outcome is that you get more extra space when the cost equalizes. This special module, as kyro explained earlier, cannot be added to anything smaller than 200 capacity. The display says this actually adds ~94 capacity to a tiny hull. This definately prohibits use on anything smaller than the flagship.
Regular module is configured as so:
Cost: 25
size: -5
sizemod: -100
range: 4
Flagship module is configured much as kyro recommended:
Cost: 5
size: 100
sizemod: -50
range: 12
So, thats how I decided to do this. Just figured i'd lay it all out in case anyone else was interested.