Hello everyone. I'm relatively new here on this site... only a few posts. 
..mostly absorbing as much as I can.
...Right now I have a 45 gallon cube reef tank going, but I'm looking to get into the planning stages of a custom motoro tank. (obviously going larger than 45 gallons). (I'm on Reef Central and Manhattan Reefs under the same name, if you're into saltwater as well).
As for my knowledge of rays/freshwater in general.. well..... I had a successful 75 gallon discus tank for about a year. It was going well, but was taken down when I moved. I've read quite a lot about their requirements, and have seen a bunch in person.
I'm very familiar with sumps, protein skimmers, and saltwater filtration in general, but what's 'big' and 'current' in the freshwater world? I used to use Aquaclear HOB filters but I want to set up a sump with some good filtration for the ray tank.
As for the tank, let's talk practical minimum here. Not trying to stuff a fish into a small space, but realistically what's the smallest you'd suggest for size? I'm thinking of aprox. 6' by 3' (by maybe 14 or 15 inch height)..
Thanks in advance for the help.

..mostly absorbing as much as I can.
...Right now I have a 45 gallon cube reef tank going, but I'm looking to get into the planning stages of a custom motoro tank. (obviously going larger than 45 gallons). (I'm on Reef Central and Manhattan Reefs under the same name, if you're into saltwater as well).
As for my knowledge of rays/freshwater in general.. well..... I had a successful 75 gallon discus tank for about a year. It was going well, but was taken down when I moved. I've read quite a lot about their requirements, and have seen a bunch in person.
I'm very familiar with sumps, protein skimmers, and saltwater filtration in general, but what's 'big' and 'current' in the freshwater world? I used to use Aquaclear HOB filters but I want to set up a sump with some good filtration for the ray tank.
As for the tank, let's talk practical minimum here. Not trying to stuff a fish into a small space, but realistically what's the smallest you'd suggest for size? I'm thinking of aprox. 6' by 3' (by maybe 14 or 15 inch height)..
Thanks in advance for the help.