we have tested drives up to 1TB, but i don't see why larger drives would not work. From software side there is nothing that blocks larger drive use.
I put a 4TB drive in my 2116 today. I originally put it in a USB enclosure and partitioned it on a CCR2004-16G-2S+ running 7.6. Then I moved it to the 2116. It turns out the 2004 created a 2TB partition and formatted that, leaving the remaining 1.8TB unused.
Once on the 2116, I went to format it again, and it couldn't do it with a partition table enabled (running 7.7). I formatted it without a partition table and it is working fine with all 3.8TB available.
I copied all the files using scp from the old SSD to the new one, but the permissions were wrong and the containers wouldn't start. It looks like I'll have to rebuild the containers from scratch (mostly the pi-hole one). There needs to be a better way to synchronize two disks across routers and maintain proper ownership for files.