Q: Mirroring Screens

I am trying to mirror the screen from my macbook air to my iMac - both are running on Yosemite. When I go into Preferences Display I only have two tabs Display and Colour - the arrangement tab is not on either computer. I am able to share screen but then that means the screen is the same size as my macbook air. I want to use my iMac as the screen for the air.