Welcome to the board brit!!!!
It really depends on you, your schedule, and your amount of dedication to learning. If you've been reading then you know that the smaller the tank, the harder to keep healthy parameters. You have a small margin for errors with a small tank. However, a small tank is cheaper, and in my opinion, if you can handle a small tank and not have anything die on you and your parameters are maintained then you can have any size later down the road. I'm a firm believer in learning to do something as hard as it can be learned, then when the time comes for it to be easy, you float right through it.
I started with an 18 gallon, and I lost quite a few creatures to my own stupidity. If I was you I would get a 29 gallon cube, or a 55 gallon, these two would be some good starters.
IMO
Clint
Of course BangGuy is a good fella' to listen to also.