Mongo's new Imac is the 21.5 inch and is set to 1920 X 1080 and is spot on. Your larger screen will need a higher resolution

go to the apple symbol on the top left of screen - then to "system preferences" - "display". Make sure of the 3 options in that window (display, arrangement, colour) it is set to "display". A window will open with a list of resolution. Mongo thinks it should be set to the top resolution on that list