The document setup menu will not render larger then 4000X4000 (At least not for me). I typically have a render size of 5179X1831. What I do is check the document's pixel size with a blank PS file and then "Render to Disk" with a document of that size. Enter PS of Paint and create a new document at the size and DPI you want and then check the pixel size. Transfer this setting into the RtD dialog and walk away from your puter for a day or two or ten. :) I've also discovered that unless your image requires one of the premium options, i.e. Depth of field, regular strength antialiasing is adequate with LARGE images. I did a test and I could see no difference at all at the high resolution.