if you want something that stays small you have a few options
potamotrygon reticulata
potamotrygon motoro "marbled"
potamotrygon scobina
potamotrygon hystrix
All of the above fish have their pros and cons.
Potamotrygon reticulata get to about 14" disc length on adult females. These guys have unaturally long tails so keep that in mind. The males stay smaller. These guys are extremely slow growing and are some of the pickiest eaters. Mine would only eat live blackworms and on ocasion nibble at the frozen blackworms. Thats it. They have cool paterns when they are young but they slowly lose them when they get older. These are the cheapest ray around. Most lfs get in tiny, malnurushed specimens that are extremely difficult to rear back to full health. I've succesfully done this twice but on multiple occasions, I thought they where going to die just because they kept refusing all foods. If you get one of these, make sure that you can feed them at least once (prefferabely twice) a day with live blackworms while they are small. They are pretty hard to keep and are very vulnerable to fungal diseases
Potamotrygon motoro "marbled", the marbled motoro. Unlike their closely related cousin, the regular motoro wich is one of the largest rays that is commonly sold, they stay very small. There are some so called "mini marbles" that suposedly stay even smaller. A large female will be 16" disc length but most stay much smaller. These are probably the nicest of the smaller rays. They have a marbled orange and brown pattern. These guys are not very finicky at all. They are the 2nd most expensive one I listed above and you'll probably have to get one shipped because to my nolledge, there are no local suppliers. These guys arent extremely hard to keep and are a great choice for a first ray.
Potamotrygon scobina. Arguabely the smallest ray, large females only reach abput 13" in disc length. Males stay smaller. These guys are just about never found in the US and I honestly dont know much about them. If you find one for sale in the US, let me know
Potamotrygon hystrix. These guys are another great choice for a beginner ray. They eat like pigs and stay small. The only problem with these guys is that is extremely hard to find a true histrix. I know rich (richone) has a true hystrix that he got from another hobbyist. Many stores say they have "hystrix" for sale that turn out to be the peru hystrix. The peru hystrix doesnt stay as small and isnt nearly as pretty. True hystrix are never found in peru.
As for tank size, make sure to have a tank with a large footprint. Height doesnt really matter much because rays spend almost all of their time cruising on the bottom. If you have any questions, feel free to ask me.