Hard to know, you'd need to know if someone has one, and that that person also has a N900. Its really hard to know if a FAT32 FS can handle 2Tb, and if the device can browse it all. What i would consider is this: splitting it in half and formatting the card 1Tb/500Gb for a desktop OS, 1Tb/1,5Tb for storage/FAT32.
That would at least limit it a bit... And, you could always try out 32, 64, 128, 258 and 512Gb partitions and see how far it can go with a kind of FAT filesystem.

If the N900 did support 1Tb... it would be the best MID in the world...