A lot depends on the type of tang and the size. For a small yellow or purple tank you could easily use a 10gal tank or a 22gal Rubber-maid container. For a larger tang you may need a 4' tank, 55gal, or a 4' Rubber-maid container.
Currently I have a small LT tang, about 3", in a 10gal tank. The tang has plenty of swiming and is not overly stressed out.
I've QT large H-Tusks in a 4' Rubber-maid contain and good size Trigger fish in a 10 or 20 gal tank.
I would imagine the size of the tang, and quantity would make a big difference in the size of the QT. I qt 1 - 2.5" pt, 1- 6" dfpuffer, 2 - 3" tomatoe clowns, 1- 5"antinatta lion, 1 - 6" marine betta in 20g.long with no problems until one of them decided to try and kill the betta. Once the betta was removed all was well again. They were qt for 8 weeks.
What are you trying to QT? Also you don't need to keep the QT running all the time, just when you're going to QT a fish. You can do a water change on the main tank and then use the water for the QT tank.
If the fish are small you could use a 10gal tank or a small container around the same size. Do a water change after you buy the fish and use that water for the QT.
If you plan on getting larger fish, and you can't setup a larger QT because of space problems, you may want to see if anyone on MR could QT it for you.