The general theory is that more water is more stable and easier to maintain. But, more water also causes more work in other ways. I have had tanks from 10 gallons to 40 and find I like the smaller ones. Right now I have a 10 but might bump back up to a 20 again some day because a couple of fish I have decided to try won't fit the 10. I personally would never go any bigger than that because the larger (medium) tank did not work for me. If I get more money to spend some day where I can get a python to help with water changes, maybe I would change my mind, but that is not happening any time soon. I have kept my 10 clean and stable just fine. There are ways to do smaller reef tanks if that is what you want, just think about it ahead of time, decide what you really want to do, and don't get talked into something you don't want.