johnjohn,
I'm am purely a novice so take this for what is worth. Like the guys said in previous post, biggest you can get away with. I like to leave at least a 1/4 inch from the end on smaller projects. I just don't like getting close to the biscuit being so close to the edge.
If this is a project that won't stand much abuse, I would go smaller if in doubt because a strong joint will likely not matter that much.