It all depends on how much space you have and how adventurous you want to get! Obviously, the bigger the tank, the better. Larger tanks are more stable and do not need as much maintenance as a smaller one.
If you really want something different, (and are brave enough to try), you could always have a go at making your own!
Most glaziers will have what is known as 6mm salvage, which is plate glass that's from shop windows that have just had a corner knocked off it, etc. It is ideal for repairing or making fish tanks with, and is relatively cheap. Obviously, you'll not be able to make enormous tanks, using this thickness of glass, but it does allow you to be a little creative, and make something quite unique. Using re-enforced glass, (the kind with the metal grill in it), for the bottom sheet, if you're going to make something unusually deep. Plus, bear in mind when making a tank that water is heavy! (10lbs/G.) So the stand it will be placed on has to be very solid indeed.
It not be the cheapest option, but at least you'll have something fairly unique!