Big as you can afford with the amount of space and money you have. It will be easier to maintain, you will be able to have more fish so they wont eat eachother and that way your not turning around in a few months rebuying stuff because of poor planning.
If your not rich I would look online for people trying to get out of the fish hobby, I picked up my 105 gal tank+ stand w/fish for $450 used off craigslist. It did take a month of watching but I'm glad I waited.