How can I copy a big file from one Windows machine to another limiting speed transfer?

Use Robocopy with /IPG switch.

http://ss64.com/nt/robocopy.html for more info on RoboCopy

Use Robocopy with a /IPG switch to limit bandwidth. Robocopy comes standard on Windows 7 and Windows 2008 Server

http://yellowtriangle.wordpress.com/2012/06/28/bandwidth-throttling-with-robocopy/

this link includes formula for figuring out maximum speed.