We can usually get by with a minimum of 16 by 12 and we have bass, lead keyboards, sax and a drummer, but I agree, a 20 by 16 or 24 by 20 is a lot nicer. I personally don't like really big stages, since it spreads out the band too much.
We played today on a theater type stage that was 26 by 24 and we had a ton of room left over. Of course, we always try to put the mains off stage whenever possible.
"A man may fail many times, but he isn't a failure until he begins to blame others" -- John Burroughs
"I consider ethics, as well as religion, as supplements to law in the government of man." -- Thomas Jefferson