of course the bigger the better, but there is no min size tank. the most important part when having an octopus is that it be kept in a sealed tank, or it WILL find its way out. blue ring octos are actually very small, only inches across, but also have a short life span. a tank only need be stable to house one, as the food that you would have to feed one would come from external sources (you), the tank by itself would not support any octo food wise (unless you dropped a bunch of living shrimp in there).