A 125 would work perfectly for pups.
Adult Motoros can clear 30in plus in disc diameter so they will need much larger.
All rays require ample space.
You could always build your own indoor pond

Save yourself the headache and build it once. 8'x4'x2' is great to start with if you have the room now. (Trust me, you will thank yourself for it. It will save alot of work!)
I am assuming you are new to rays.
Remember when it comes to keeping rays research, knowledge and dedication are key. This forum is a great place to start.
The stickies here have some information to get going on. After that I would recommend literature by Dr Herbert Axelrod, and Baron's. Both books will answer many of your questions, as well as inspire many more.
Keep us posted on your progress!
Good luck and welcome to MFK!
