I just found out that there's a little bug in the save as dialogue as regards to the resize "to fit box" option.
Set it to e.g., 700 pixels. Save a snapshot of these settings. Now change it to e.g., 5 inches, 300dpi and then load the previously loaded snapshot. All seems ok, but saving an image this way won't work as somehow it still thinks it is saving in inches, not in pixels (even though the dialogue says pixels). A tell-tale sign is the decimals added after the 700; there can't be fractional pixels of course.
Probably something that can be fixed easily