The biggest ones there are go in there. Many years ago, there was a 2TB limit in the QNAP TS-209, but that has long since fallen. There are no longer any such restrictions with the hard disks.

My tip for those on a budget: buy 4 x Seagate external USB hard drives 5TB. Then open the housing (glued or plugged in, super easy) and "harvest" the 5TB SATA Seagate Barracuda inside. They are in my QNAP, super quiet, hardly need any power, no vibrations. And they last for years in a RAID5. With approx. 480 Swiss francs of hard disks, you can already get 15 TB of usable content :).

Each Seagate Expansion Portable 5TB USB enclosure contains a Seagate Barracuda 2.5 TB notebook hard drive with a height of 1.5 cm :)

Seagate Expansion Portable (5TB)