The smallest ray is generally considered to be scobina. That is generally followed by histrix and reticulata (magdalenae).
The down side is that scobina and histrix are from Brazil and they are very hard to find these days. If you do, they are $600+.
Retics are common and cheap but, they still grow to 14" in disc diameter and 30" in total length.
The common opinion is that no ray is suited for any length of time in a tank smaller than a standard 180 (72"x24"x24").
A 75 gallon is a great grow out for a pup. It would last no more than 1 year.
Colin