FW stingrays are what got me into the hobby! they are amazing creatures, I've lost 2 stingrays over the years, and before they passed on they actually waited for me to get home, swim to the tank, and then stop breathing
Over the years, I've had 5 stingrays in total, 3 lived for 3 or more years before i had to let them go because i had to move. With regards to tank size, i think it depends, I've had success starting them off in 10 gallon tanks, while constantly upgrading every 3 months ( i kept them in small tanks coz i was in a rush to get them as well as grow them), in retrospec, getting a bigger tank for them would be optimal cost wise, but doable in smaller tanks-----the only downside though is that WC was done everyday (if it was small) as well as u need to be bare bottom, and scoop/clean up after every feeding or pooping. IN the end i ended up with a 150 gallon tank.
In terms of rays, ironically, the hystrix (teacup) which is the smallest of the stingray group is NOT for beginners. They are probably the hardest to keep from my experience and my old group of friends experience. The motoro (brown in color) often referred as blue/orange one is probably the beginner ray for me.....with regards to disc size.....im not sure how big they get, mine got to around 20-24 inches in disc diameter. I do know though that u need around 36 in width aquariums to have them live comfortably.
This is the stingray thread in mfk:
You can find all the important details below there
In terms of feeding them, I like feeding them guppies/goldfish/ghostshrimp once in a while
keep them on top of their toes. But that is a bit risky unless you know where to get them, i.e. breed them urself, or have a friend who breeds them. Normally i rotate their diet during the week, and they consist of the following; scallop, shrimp or prawns, and mussels (all deshelled), as well as bloodworms. I MAY in a sense be spoiling them......haha but yeah thats' their diet. Its easy to get them to eat from your hand. On a side note though, the rule of thumb isn't "if it doesn't look like it will fit in their mouth, they can be placed with the stingray. I once got a frontosa before, and i believe that there was no chance it would fit into the ray's mouth but when i awoke the next day, the frontosa was gone, the stingray was happily burried under the sand, with a huge bump on its stomach. I think the rule is.....if it can fit under the disc...think again.
Hope this was helpful