That depends.....

Windows 9x/Me uses the BIOS for HD access, so whatever size your BIOS supports is the biggest you can use. Probably, as ludicrous said, 2.1 GB is the limit...although you could get lucky and be able to use an 8GB in it.
If, however, you were to move to either Windows NT or Linux, you wouldn't have that problem, as both of these access the drive directly, and, once booted, can access any size of drive. I've got a 20GB drive running on a 486 with NT4, and I've had similarly big drives running on 3/486's with Linux.