Check out the shipping size on Ebay auctions. Some vendors will list a size.
http://ecat.spectrapremium.com/a/000061710
Spectra says the 18 gal tank is 29 3/4 in. x 17 1/4 in. x 13 5/8 in.
Their 23.5 tank is 29 3/4 in. x 17 1/4 in. x 13 5/8 in.
The bigger tank needs 3 straps to hold it, the smaller tank uses 2 straps.
If you have a long bed (you do) the bigger tank just slips right in - the factory did it and so can you.
Actually the 23.5 gal tank is 36-1/2" x 17-1/4" x 13-5/8 it is 6 3/4" longer which is why it uses Three straps.
DT