Well, this is kind of hard to measure, as it is behind a very high quality optical lens system. I am guessing it is about the same size as. 2/3rd to 1-inch Sensor. It is very easy to see, gives an almost 1:1 image size when viewing it. Since you do not view the screen directly, as you would less expensive EVFs, but rather are viewing it through a high end optical lens system. It is on a par with the Zacuto Gratical EVF, and probably a similar size, but at a higher native resolution. It is also a OLED display, and very easy to focus with.
Cheers