I used to keep fish at my parents but now have a place of my own. my girlfriend knew i liked fisha and got me a 2ftx1x1 tank for my birthday... she wasn't too keen on the idea of fish but she is now hooked. We went from the 15gal to a 50 gal in a matter of months (4 to be exact).
we then set the 15 gal back up as a 2nd tank.
another 6 months down the line and we're looking at our 3rd tank
Large tanks are always easier to look after then small, they are more stable, things happen more slowly. If you have the space, and the cash, why wait.
Go for the big tank as soon as you can. Once you have learned the basics of cycling and managing a tank, you will be able to keep any size tank you would like. All the same rules apply, whether your aquarium is large or small.