Modern NAS such as the Synology DS1821+ can accept all kind of sizes, there is no limit, as big as you can get, they will work. In th past, there were NAS systems that had a 2TB limit, but these days are all gone. At the time the Synology NAS supports 16TB drives, but that is only a matter of time, if larger will become available, this number will go up also.

No, the DS1813+ is too young to have such problems. IT supports 108 TB. All the information how much can be added is also found here:

https://kb.synology.com/en-af...