Go as tall as you can afford to make it, the extra volume would be very beneficial in helping to maintain proper water quality. Plus it gives you more options for tankmates.
I'm not a ray expert but, 14"-18" would be fine.IMO.
Wall swimming is not a natural behavior but, on in captivity since I don't think they run into glass walls in rivers! LOL.
that being said footprint is way more important. The smithsonian zoo has an amazon exhibit with large motoros and Leo's in a lagoon that's not over 12" deep. Most of the rays I saw there were in 4" or less of water cruising around.
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\one of my female marbles has an 8" disc another 9" and another 10" they are the females.
did anyone even ask what size or type of ray the op is getting ?