The latest issue of the UK magazine MacFormat (Issue 215, December 2009) I have has a "Group Test" of six portable hard drives. That's by no means an exhaustive list, but the largest size option for all of the models was 500GB, with the exception of the
Seagate FreeAgent Go for Mac which has a 640GB option.
Here's their review ratings (out of five stars) in case it's any help:
4 stars, Buffalo MiniStatio Lite (USB 2.0)
2.5 stars, Freecom ToughDrive Custom SE (USB 2.0)
3 stars, Iomega eGo Portable Hard Drive (USB 2.0, FireWire 400, FireWire 800)
3 stars, LaCie Little Disk (USB 2.0, FireWire 400)
3.5 stars, Seagate FreeAgent Go for Mac (USB 2.0, FirWire 800)
5 stars, Western Digital My Studio Passport (USB 2.0, FireWire 400, FireWire 800) - Group Test Winner
Edit: I've just read the reviews and the introduction does say:
"On a [UK]pound-per-GB ratio, portable HDDs are by far the cheapest way to carry around large amounts of data, and they're available in sizes of up to 1TB."
but none of the six drives they tested have sizes that large listed in the specs table.