The screen resolution specified in the xvfb-run command is 1280x1024, so the grabbed screenshot will be 1280x1024 as well. You can change the resolution of the emulated screen or crop the image using parameters on the wkhtmltoimage command. Quoting the wkhtmltoimage manual:
--crop-h <int> Set height for croping
--crop-w <int> Set width for croping
--crop-x <int> Set x coordinate for croping
--crop-y <int> Set y coordinate for croping