We have a few small welded wire rings, roughly 3 feet in diameter and 4 feet high, that we use around the bird yard. That's about the minimum size to get the piles to heat to the point of being able to kill off most seeds that make it in there, and it's a simple thing to lift the ring and move it over so the pile can be turned. You could make it smaller if you're not too concerned with volunteer plants sprouting. I know a few folks who will line the inside with landscape felt (not the plastic stuff) to have less material fall through the gaps in the wire and make them look a little cleaner, if that's something you're interested in.