It also depends on the fairway layout of the marina as well.. marinas with narrow fairways may need wider slips to safely accommodate boats turning in and out of them.
I've been to marinas that don't allow a boat to poke out beyond its pilings. Other marinas don't seem to care. I've seen plenty of boats that poke out of their slips get snagged by other boats, so I generally prefer a bigger slip so I can be a little deeper in. The downside is that the pilings are too far away to grab when working docklines, but I deal with that.