Like Mitchel says, a lot has to do with just how big the file is when it's uploaded. I've done 100MB+ files but when I've done them it's been on the internal network to the webserver so it was going at 100Mb/s vs the usual 10Mb/s that my internet connection maxes out at. Even then I had to boost the executionTimeout in my web.config. If you upload the file via FTP then the way Mitchel suggests is probably the easiest. Upload a small test file (a notepad created file will work fine). Then look in the File Manager or check via FTP to see what the file has been named and rename your file to that name and upload it. Or if you've already uploaded it then as long as it's in the correct directory you can just rename it through the File Manager (after deleting the test file). If it's not in the correct directory then you can try moving it via the File Manager but I don't know how well that'll work. The next step would be to manually fix the file size in the grmRepositoryObjects table.

Note, the current version of the Repository is limited to the portal having < 2 GB of storage. That's something that is fixed in the next version which is currently undergoing testing.