Dirt walls, not stone? Really you just need to be able to stop their movement across them, so 1 raise wide and 2 tall will do it, but 4 tall is better. In either case you have to use your pick to carefully trim along the outside foot of the walls to make the side as sheer as possible so nothing can walk/run up and onto it. If you have the stone then 2 wide is better because its easier to make it pretty. Note that if at all possible your walls should be north-south and east-west to align with the grain of the world to look tidy. (Its hard to make the sides sheer enough to stop things walking onto them if theyre not aligned with the grain, as-it-were.)

Obviously you need to make them taller if you need to stop them throwing rocks too, but whether or not you need to stop rocks is a matter of individual base design