You've got two kinds of hammers. Wall hammers are usually a little harder to keep but they grow out horizontally. I don't have much experience with those. The other kind is branching hammers which are usually hardier and easier to frag once the branches give you enough length to cut them. They stay pretty round in shape though.
To give you an idea, I gave mine about six inches on all sides which is more than enough for two year old hammers. I have a monti cap that grows out close to the hammer every now and then and if it gets around three or four inches from it, I have to frag the monti back because otherwise the hammer's sweepers will sting it and start turning it white.
