The bigger the better is pretty much the rule of thumb. More water is easier to control and keep stable. In the next 2 months you owe it to your future fish to read every article and bit of information you can on how to keep fish as well as read up on the fish you plan to get. You want to be an expert as far as knowledge goes on the fish species you are going to get.
Definitely more than anything else read the article about the nitrogen cycle. Reading is the best thing you can do before you get a fish tank. Before is the key word. Also once you start to get closer to having an idea of how large a tank you are going to get, ask around here for whats the best filter types (or better yet, do a search for filters in this forum and read what's already been said) Please read everything you can, it'll save you some headaches later.
That being said, I don't know much about angels and I'm not equipped to give any advice