What size sump????

[allincuddy]

I am setting up a 120 gal. tank with 120-150 lbs of live rockand a 4" bed of sand, the tank will be filled (hopefully) with all kinds of different corals and only have maintenance fish and cleaners only.
My question is:
How big of a sump do you think I need, is a smaller sump, just to house pumps, skimmer and ca reactor with an overflow drain incase of power outage sufficient.
What would you do????
Smaller sump meaning 33 gal.
Please any adivce would be welcomed.
Thanks in advance.