If you want to be really safe, assume it's 1 TB, because that's the largest reported to work.

If you want to be a bit more adventurous but still be safe, assume the limit is 2 TB, because

1. there is a variety of legacy design decisions for computer storage that gave rise to the 2 TB volume size limit (any of which independently may limit the volume size to 2 TB, though it's possible that none of the affects the PS3)

2. We know it is possible to have 500 MB and 1 TB HDDs on the PS3, and the next plausible limit above those is 2 TB.

I believe the biggest HDD that still fits into a PS3 is only 1 TB (PS3 needs 9.5mm or smaller HDDs. The 2 TB WD20NPVT is a 15mm one), so the above 1 TB limits are only theoretical at this point.

"I dont buy Steam games because when I buy a new PC, I have every game I ever bought already ready to download." - GoreGamer