I think the biggest is currently 750Gb. They are coming out sometime in June for about $150 I think. But if you need one now, it's 640Gb. It's a Western Digital one. Don't forget the hight needs to be 9.5mm, not 12.5mm.
The dimensions of 2.5" wide and 9.9mm tall are limits but the capacity of the drive has no limitation. If next year at this time there is a 2 or 3TB 2.5" SATA 9.5mm tall drive, it will work.
Please don't forget that when your internal drive increases, the size of your backup drive needs to increase as well.