So you were planing along at good speed but had spin outs now and then? The problem was not that the board spun out on you while trying to gain speed?. In the former case the problem is normally not the size of the fin as the other indicate. In the latter case it can be. Can you specify the nature of the problem a bit more, ie in what sort of situations you get the spinout (and also where you run the straps + if you run double strap in the back).
I'd say that unless you're heavy or heavy footed, the 30cm crossover should cover a 6m2 sail quite well. It's a pretty powerful fin (I run the 28cm version up to 7.0 but I'm quite light).
I agree that 30 or maybe 32 is around the max size a US box will take. But the box installations are much, much more reinforced these days. And the shorter the box, the stronger the box itself will be (assuming the fin base length stays the same).