I have 15" drivers. The Hitchhiker's Guide to the Galaxy ended up pushing the grilles off my top-facing drivers (I have sealed sonotubes with drivers at both ends). So this past weekend I ended up redoing the legs and replacing the top grille with the legs and base as well. So now it looks like you can flip the sub either way. I went with 4" legs because I couldn't find screws any longer than 6" at Home Depot.
Otherwise I was going to go with 5" legs.