Files several gigabytes are too large. You need to compress them down to file sizes in the 50-150 megabyte size. YouTube has a max video length of 10 minutes. I don't think they have a max file size limit though, although whatever you upload gets encoded to flash.

I use Quicktime compressor, which although it doesn't have too many controls, it's easy to use. Apple has Quicktime for Windows, a free download.