I attended a talk by one of the foremost ray keepers in Canada not long ago.
She is the curator of rays at the Vancouver aquarium (at least she was at the time of the talk) and spends a lot of time in S America with rays.
One of her recommendations was not using any tank under 300 gallons for even the smallest rays (although she said foot print was more important than gallons).
She was very personable, and I would imagine if you contacted her (sorry don't remember her name) at the Vancouver Aquarium, and explained your interest, she could be full of great info.