I think 1TB is probably the max. I put a 500GB in mine using the network adaptor a number of years back after coming across a page with a PS2 Compatibility Chart. Check this search HERE. Searching for 1000 as the size will show the 1TB. I didn't find any larger, but the site doesn't sort that great.
New missings
4x4
Minions Pinball (Original 2017)
Rapid Fire (Bally 1982)
The Table With The Least Comprehensible (Original 2017)
The Pinball Food Fight
Super Nova (Gameplan 1980)