I have a 55 with a 30long sump, I chose this to leave 12" on one side for my supplies. The larger the gallon capacity of the system the more play room you have for live stock and invert selection, not to mention just room for various equipment generally housed in the sump (protein skimmer, UV sterilizer, bio-filter media, and retrun pump). Larger sumps also leave room for a future refugium, which adds loads of inherent biological benifits. So I guess the answer is as large as your space and budget afford. Just my opinion anyway.
Hedonic