i read that it was height and then i read in on a DIY sticky that it was width so i was confused, but with this in mind as long as a tank stays 24" tall you can use the same thickness of for what ever foot print??
ha never mind i just figured out y after reading this (what i had originally read that said glass thickness determined by height.) http://www.austinglass.com.au/aquariums.html
ok this is was i came up with, so water pressure is basically the weight of the water per unit area so lets take lbs/in^2. well so a 4"x4"x4" cube of water weighs roughly 2lbs. and is exerting this force on a 4"x4" area (the foot print) or 16in^2 area. so the pressure is 2lbs/16in^2 = .125 lb/in^2. ok now lets double the length 8x4x4 (doubling one side wil double the weight of water) so now the weight is 4lbs but on an area of 8x4= 32in^2 so 4lbs/32in^2=.125 lb/in^2 pressure is the same. ok now lets double the height so 4x4x8 the water weight will double so 4lbs but the area the pressure is on is only the original 4x4 or 16in^2 so now 4/16=.25 lb/in^2 so the pressure has gone up. so now that is the y the height matters and not the foot print man i am glad i just realized this cause now i can buil my 270 gal. these are the calculations that prove y height is what matters in determining glass thickness not the size of the foot print.