Maybe about 8ft in diameter- roughly 750 gallons. I believe that's about the size that Crumbs(Chris) has for his Short-tails.
So in the long term - it's easier to keep a pair or trio of Short-tails than it is a single adult Atlantic Nurse. Short-tails require a lot less space, less water, and less food.
By comparison a single average(8-9' in length) adult Atlantic/Common Nurse would require a pond - that's at least 570-600sq.ft. (footprint), over 17,000 gallons to keep it for the rest of it's natural life(which could be another 20 years or so). And you could easily figure at least 400lbs of seafood (shrimp, crab & fish) per year.