Word of warning though, if by landscape rock you mean mini-landscape rock or seiryu stone, it usually fractures to give dull and uninteresting semi-conchoidal faces of a darker colour. The contrast can be a bit jarring and unnatural. That said, it is possible to avoid that to an extent if you're careful to select pieces with existing fracture lines that lend themselves to being split.
I always try to avoid breaking it up, but sometimes it's necessary to get the pieces to fit. Like Stu said, use a big hammer. I've also variously used an angle grinder and hammer and cold chisel
Overall though I always find it's best to buy smaller pieces if that's what you want.