Those drives are so cheap anymore I would just go ahead & try it. The worst that can happen is that you end up with a cheap backup drive.
The 8gb file max size may be inaccurate due to older OS's possibly being used or could be an actual constraint from the amount of memory in the router(There could be settings you can change or even just a bios update to fix some of that too).
If you're the only 1 using it then you won't be fighting for bandwidth, so you can see how much they fudged their speed #'s on the box compared to actual use...
Just remember that you'll be using a router USB port as a NAS & that's not it's primary purpose. If you're that concerned with the speed & file & drive sizes, go get an actual NAS. You can get a cheap 1 to do what you want and not have to worry about it.
Hope this helped a bit...
& Good Luck!
Pessimist: Oh, this can't get any worse!
Optimist: Yes, it can!