If you have ticked the box underneath that standard uploader in order to use the Flash uploader, then the largest file you can upload is 2GB. This is due to a limitation within the Flash file uploader.
You may also see problems if you have an unreliable network connection.
If, for some reason, you find that you need to reduce the size of the files you are uploading then utilities exist that may help. For Microsoft Windows these include Winzip. On *nix systems the split utility can be used to split a file into multiple files, which can then be recombined with cat.