Example of a game that works nicely on the deck, and is giving me a great portable replay (other than the EA Launcher): Mass Effect Legendary Edition.
Size of that game: ~110GB
Also, personally, because of the slightly shonky WiFi download speeds and that I use it offline/whilst travelling a lot, I leave a bunch of games on there just in case I fancy them.
To me, more space would be good. I am unsure I’d need 2TB, but it would solve a lot. I’ve also got a few hundred gigs of other launchers (battle net) and ROMs on my SSD currently that are less easy to manage than Steam installs… again, the larger drive would just lighten the worry.