You should start any research into a cruise by deciding where you want to go and what you want to see. For instance, if you want to explore remote islands in the South Pacific then a large ship (over 300 pax) is unsuitable. If you fancy cruising the Mediterranean then bigger ships are OK because each port of call will probably have port facilities. If you just want to cruise for cruising's sake and merely enjoy the facilities a large ship can offer then research individual lines.
I have done one cruise each with Seaboard and Silversea on ships with no more than 300 pax. Food on Seabourn was markedly superior in my view but otherwise standards were roughly the same. In both cases, pretension counts for more than genuine quality.