What is the minimal tank size i should get
Sorry dude, but thats a bad question to ask. (Many others on here will agree)
But i'll still try to help, first of all, dont get a tank that will "just" hold the fish, get something that will allow the fish to breathe and grow without being cramped, if you are getting something small like a Hystrix ray then at least 2.5' wide will be needed, and at least twice as long.
The tank doesn't need to be high though for rays. If you are getting something bigger than a hystrix a bigger tank will obviously be needed.
Many people put arowanas with Rays, Pretty much anything that is a top swimmer will be ok, bottom swimmers will cause too much competition. And nothing that can fit into the rays mouth.
How long have you been in the hobby? as rays are not a very good choice for a beginner.