As big as you want - it just depends on how you want it to look?
But - the common 'wisdom' - is 1 - 1.5 lbs/gallon - so I guess you could say most people would say 30-40 gallons? It depends on so many things - what you want to have in the eventual tank (i.e. corals grow big - and will make 'more rock'. If you don't want corals then it comes down to which fish you want - big swimmers or big hiders. There will be lots of opinion on this - I'm not sure there is one answer to the question