hey
generally the bigger volume the better, so as mr B said, the biggest you can afford really. it is an expense to start with, a typical system will cost on average around £20 per gallon
what i would suggest is when it comes to dimension, go for wider than taller, the reason being, its best to have as large a surface area as poss, so basically a tank say 4 foot long x 2 foot wide x 3 ft high would have the same capacity as a tank 4ft long x 3ft wide x 2ft high, but the second option gives more surface area than tank one, which would give you more stocking capacity
space is another consideration, a typical 120 gall tank would be 5ft x 2ft x 2ft
big in itself, then the stand and then the hood or lights, i have this size tank in a fairly large room and it still dominates a good area.
weight is yet another important factor, i cant remember what weight i worked my tank out to be, but its bloody heavy, if you live in an upstairs flat, or plan to have the tank upstairs, then just be wary.
there is a mod from another forum i know, who got the builders in to put extra supporting walls and a few other building works to support his (i think)
250 gall tank
anywho, if i think of anything else i'll get back to you
big is beautiful
stay fluffy
ty