well we cant answer what tank size you should have until we kn ow what seahorses, and how many you plan on having, or vica versa.
you can keep seahorses in 5 gallons, or 5,000 gallons, it is realy up to you on the size tank, their will almost definately be a seahorse that will fit it.
the acrylic needs to be to the specifications of your tank, the larger the tank, the thicker the acrylic, are you building your ownb tanmk???
as for dimensions, remember the taller the better. if you only have room/money/other factors for a tank under 50-60 gallons, and you aren't keeping hippocampus zoostera, than i would spring for a tall tank, as the tanks get larger, the long ones are usualy tall enough to support seahorses.
hth
