Those could be the recommended screensize for readable text, with a small border of 2x8 pix horizontal and a 2x23pix vertical border.
Though I prefer text without border, since it takes up screen space, and you already have a border around the screen. (I mean it's not like we can make annotations on these devices or so).
when I load a pdf,with this info, and zoom on text width, I notice that the left,top and right border are the same size.
So let's assume you have a border of 8pix left,up and right of the text.
This leaves a border of 38 on the bottom.
take 13 pix of the bottom border since it seems a little larger than the others, and you'll have a bar on the bottom of about 25 pix.
in other words it could be that images with a size of 775x600 are displayed exactly the same as 800x600.
In order to figure that out I can try to see if I see any difference between the quality of 800x600 and 775x600.
Obviously 775x600 will be more stretched looking.
However I had preferred if someone knew by playing around with the internal software what the exact resolutions are.