The bigger the tank, the better.
There is a minimum recommended size, but not maximum, that is because, the bigger the better.
When starting out, its important to do things right, and with a big tank, you will be far more capable of keeping a stable environment than in a small tank, small tanks are a lot more work that large tanks.
Also, seahorses do love room to play, and in a big tank, you can add lots of interesting things for the seahorses to explore and hunt, meaning, far more activity, and so, more fun for you.
Captive bred from the likes of Dan (seahorsesource) is very high on the list of priorities.