Do you have a limit on width or depth?
A container which is 24"w x 24"d x 6"h = 15 gallons
To play with figures go to:
http://www.worldfishlink.com/calculate.htm
That is where I go when I am trying to figure out volume for my fish tanks.
Just in case anyone is wondering, one gallon of water weighs 8.34 pounds.
One cubic foot of container will hold 7.5 gallons of water.