The issue with keeping a shark or ray, even a small one like an epaulette, bamboo or cat can be boiled down to one question: Are you interested in having the shark sustain, survive or thrive?
If you want it to thrive, at least 500g or 250 (oval/circle) and HEAVY filtration is needed. I know people say all you need is a 75 or larger, but that is simply not the case for these animals to be happy.
If you want it to survive, at least a 180g, preferably with rounded corners and an unobstructed sand track (place rocks in the middle) will be ok...but still HEAVY filtration is crucial. A large sump or fuge would be of tremendous value as it would help stabilize the water parameters.
If you want it to sustain until it dies (months/one-two years)50-180g aquarium would do. Skimming or filtration is no longer a primary concern, but still important.
I do not mean to discourage you but I need to emphasize sharks and rays are fundamentally different than bony fish. We tend to learn about damselfish, tangs, angels and other common bony reef fish and try to apply that knowledge to sharks and/or rays. While there are some similarities, there are important differences in the animals needs and anatomy that must be considered when deciding wether or not to host them in your aquarium.
I do not mean to sound harsh, you are doing the right thing by asking advice and having a plan to give the shark to your local LFS when it is too large for your tank. I just wanted to point out a few things. Just for the record, I have managed tanks for various shark species (from cat sharks to hammerheads).
Understanding them is critical to achieving your goals in experiencing and caring for them. A good source for some basic rules is
http://www.wetwebmedia.com/sharks.htm
Best of luck
