Ideal box size?

I currently have the Rockford Fosgate Prime R1L-2X12 which are 2 R1 12's that come already in a box. The only problem is the bok is on the smaller side and I really have trouble hitting those really deep lows. The subs flex quite a bit but the low end just isn't there. So my question is would getting a bigger box help me hit those lows better? If so, whats the optimal volumes for the box?

Building a box to spec. will make a hell of a difference.. look up how much cubic feet they recommend for each sub ported

You say ported, but it's recomemded sealed. That make a difference?