Not sure exactly, but the ones that I have seen around here are about 20-25 feet across. Hope that helps...if not specific enough I am sure that there is lots of info on the net.

We had one in at our last place that was probably 60 feet (20 meters) across, and it was just getting going. But I also see them spaced a lot closer usually. By the way, they're not actually maples, but are in a different family altogether.