the passive radiators serve the same purpose as ports; they tune the cabinet.
so long as the radiators are not hitting their limits, there is little reason to use larger ones, just as if you are using a port that is sufficiently large in diameter, there is no reason to use a larger one. it won't increase performance.
since they were used in a small cabinet that is probably what i would suggest.
something like 2 cubic feet for both drivers. then use a pair of 12" passive radiators on opposite sides to reduce cabinet vibration. something like this might work for you for a low cost: