The only possible way is to do a bit of trial and error to get the size and proportion you like. Here is a bit of math for you.
Take a measurement or a rough number for the length, width and height of a tank size you are thinking about and then times them all together and devide the answer by 1000.
Assuming you aren't from the US this will help.
Example:
(L x W x H) / 1000 = tank volume
so
(20cm x 15cm x 15cm) /1000 = 4.5 Litres
If you are looking for dimensions for a tank of 6 Litres then you just increase one of the (L), (W) or (H) and recalculate the volume.
(20cm x 20cm x15cm) / 1000 = 6 Litres
I hope this helps, as far as I know this is the easiest way to figure this kind of question out.