What you need to remember is that sacs, when filled, exceed their stated dimensions. I can say this, i don't need anything bigger back there nor would i want it.

For boarding they are more than enough for us. Nice big wake.

For surfing they are great, i can get it to the rubrail with my normal surf crew and water comes in on the dipped corner.

If you really think you need that much with those dimensions then Fly High will custom make them for you.