I would say that it's impossible to advise on size of tank, it's pretty much down to you really. Of course it goes without saying that the bigger the tank the more maintenance it requires, i.e. bigger water changes.
I have a 34g tank and I'd love a bigger one. As Donna says, set your budget and work around that. As you're probably no doubt aware, tanks can be very expensive so this should really dictate what you go for. If money isn't an issue, however, go for the biggest you can find. Woohoo!
As I've recently found though, the supporting equipment is vital to a happy community so big tanks need big everything else.
I'm sure you'll make the right decision either way.