My rose has been hanging out im my tank not splitting for about 22 months now. I couldn't possibly bring myself to try cutting it in half. I guess if I ever do get up the nerve the machette would be the right tool though.
If anyone does decide to cut up their big rose I would pay admission to see it.
If you want it to get smaller, stop feeding it. The anemone will actually digest some of it 's tissues for nourishment. I never feed my Haddoni or my Heteractis Magnifica so they are sort of in equilibrium with respect to growth. Occasionally I will feed them a frozen shrimp or to and they get some of the fish food that lands on them. But mostly they live on "light".
I think my rose has been sneaking out of it's tank and raiding the refriderator at night. I can't prove it, but even if I don't feed it, it seems to keep getting bigger. Don't figure???