If one of the systems is running server you can\should setup a vpn if not ftp is the way to go. There are many freeware software solutions for a ftp server, or you can use the built in one from microsoft if you os supports its. it will not be faster then mail but you will not have the size limit.
What type of file is it? there may be a better answer then sharing the full file, for example an excel sheet can be setup as a simple acess database with a sql backend on a server on one side then with a vpn setup you will only transfer the data you need to work with.
Or maybe a vpn setup and run remote terminal connection like terminal server set in admin mode or a freeware software like VNC ( http:\\www.realvnc.com
) this will allow you to connect to a remote system on one of the two sides and work on the same local copy. it may be faster to connect and work on a remote terminal then transfer the file.
I would need more information to make a better suggestion