OLED scales well to smaller sizes. The view-finder in NEX-7 is less than 1 inch wide, with 2.4MPixel resolution (aka QWXGA 2048x1152).
http://en.wikipedia.org/wiki/Sony_NEX-7
And yes, they go higher too:
http://www.dpreview.com/news/2012/01/27/MicroOLED
Those appear to be back-panels based on their size, by view-finder I was referring to the little view on-top of camera that is normally optical - but it's being replaced in the mirror-less high-end cameras with electronics.
As for back-panels - an average point&shoot these days comes with back-panels of 1-1.5MPixels, at 3-3.5inch (so 400-600PPI range).