Dell Monitor/MacBook Pro Retina Settings
What is the best setting for both my 2015 MacBook Pro Retina and Dell U2414H monitor to ensure the image on the Dell monitor is excellent?
MacBook Pro with Retina display
http://osxdaily.com/2015/08/27/show-all-display-resolutions-external-screen-mac/ This should provide the information you need, hope this helps!