All sizes of the ancillary shield boosters restore a set amount of shield no matter what size of cap booster they are loaded with. Therefore you should use the smallest size that will fit. Standard cap booster charges will fit 7, faction will fit 9.
Small Ancillary Shield Booster - CB 25 - 52 hp / 2 seconds
Medium Ancillary Shield Booster - CB 50 - 146 hp / 3 seconds
Large Ancillary Shield Booster - CB 150 - 390 hp / 4 seconds
Extra Large Ancillary Shield Booster - CB 400 - 980 hp / 5 seconds
These hp repair numbers are significantly better for the shield boost bonused ships. (duh)