The answer is hard to give, cause it all depends on what type of seahorse you can get. Seahorse Fanatic is the right for a basic setup, I would like to just expand on it a little further. Most seahorses will require three times their body height in tank height. Now bare in mind that seahorses range from the tiny pygmy's of less than an inch to the large H. abdominalis at around 12 inches. Most common to the aquarium trade are the kudas, hyterix and barbouri, all ranging in the 5-8 inch range. Now if you want to raise the fry this opens up a whole new can of worms and the best information on doing this can be found in the two link Beverly supplied.
My own experiences are with barbouri, which I had to give to a friend as they weren't eating in my tank (its a pig now, I hear), and dwarf seahorses H. zosterae. Which I keep presently in a 10 gallon hex tank and will be moving them into the new minibow 7gallon with 5 gallon sump/fuge setup, we just finished for them.
Links to pics of the seahorses - remember these are all in the 1-1.5 inch range.
http://albertaaquatica.com/index.php?showtopic=4313
and pics of the new corral for them.
http://albertaaquatica.com/index.php?showtopic=4709
Hope this helps
Paul