Too big for what? Doesn't fit in the tank, or you just don't like the large look of it? I suppose you could take a hammer and try and break it up, but there's no telling what you'll end up with.
first try breaking it with your hands...a lot of times you can break waterlogged live rock pretty easily...otherwise...bring out a chissel and a hammer and try to get it as close to teh shape you want as possible...don't use rusty tools
I used a hammer made for rock work. Had it this summer for working with blue stone. Worked well for me. I then chipped away for the shapes I wanted, rinsed in slat water and put back in the tank.
