Potamotrygon Humerosa stay fairly small, the problem is there are a lot of different rays that fall under that name. I haven't seen a true humerosa (what I consider one anyways 'P 46') over 18", not saying they don't exist, but it would be safe to say they don't get too much bigger than that. Still I would say minimum 5' x 30" x 24" (Which I think is the smallest tank for any adult ray).
Magdelenae have a fairly small disc size, but long tails. They tend to spend a lot of time swimming in the current and I feel require a lot of swimming room.
Motoros are one of my favorite rays come in a wide range of colors and patterns. They get big though, like 24" disc, so they need at least a 3 foot wide tank.
Ocellata, well I've never seen one in person and don't know many people who have them. I've heard they only get like 10" wide, but I haven't seen it with my own eyes or have heard it from a good source, so I don't know how accurate that is.
Reticulata is just a name that used to be used for a variety of rays... usually magdelenae.
Marbled.... that could be a lot of things from motoros to motellos?
Honestly, I would spend the money on getting a proper tank and then get something like a motoro to start with. When you will be spending potentially $800 + on a scobina, this might actually be the cheaper route.
Good Luck,
Justin Morash