I would first suggest that you consider keeping a couple of seahorses that are the same gender for a year or so before you jump into breeding them.
Textbook answer is that if the seahorses breed they require a minimum of 3x their extended adult length. Depending on health, your kudas can reach a length of 6 or 7 inches (maybe smaller, but overestimate for their sake).
I personally think that you would be kinder to go as tall as you can and find appealing. The waterline is equal to a headboard, or falling onto the floor...sorry for the crude reference.
You can get creative and make good decorative use of the tall tank walls, perhaps syliconing (sp?) ledges and propogating their natural habitat of mangrove roots (the leaves have to be above water).
Breeding seahorses is a huge commitment of your time and is not a seasonal hobby. Seahorses become attatched to their mate and are monogamous and, given the right conditions, will breed frequently.