My wife has a 27" iMac and the dimensions of the actual screen are virtually the same as the monitors: 13" x 23.25". I just measured it to make sure. The iMac seems to have 0.25" more width, but that's trivial.
Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5
