Size: If you're just using it to save occasional doc and pdf files, 1GB or 2GB should be sufficient.
Price:
- 1 GB: $27-$35
- 2GB: $35-$45
Durability: I think this mostly depends on the brand, but I've found Kingston DataTraveler (which they sell at titles!), PNY Attache and SanDisk Cruzer to be the best. I even accidently put my PNY Drive in the laundry and dryer and it still worked! =P I found that in terms of speed (on USB 2.0), Kingston is the fastest.
I wouldn't recommend Ultra "FlyDrive". Four months after I started using it, the drive wasn't recognized on any of my computers (OSX, XP and Vista). Not to mention for some reason, it hogged three drive letters when I first plugged it in.
Misc:
DO NOT get a drive with "U3 Technology"! U3 drives contain annoying, useless add-ons like flash games and web browsing that require a launch pad and additional software to work. In essence, before you can use the drive, you have to install the U3 software, which I can't guarantee will work on public computers. It's even more frustrating trying to get it to uninstall on Macs.
If you're prone to losing the cap from time to time (like I am), consider the SanDisk which has a sliding usb port with no cap.