At the risk of being redundant since you found a solution. Assuming you mean a way for characters in game to resize something, I would assume it's a craft check of some kind as they'd have to partialy rebuild the weapon.
As they have a number of re-usable parts they should probably get a reduction on costs but would still need enough skill in craft(advanced firearms/explosives/whatever).
Also there's a question on access to smaller rockets to go in it so magic resizing is probably the simplist solution.