Yap, my bad. I got it in the and. It must be choose an approximated Custom size from that list (in my case - 1280x720) and then my size 1280x536 in the boxes below.
Another problem with this Custom resize: it seems to let artefacts after conversion. A green line (that green line...) up the movie screen, and some strange lines at the bottom of the screen:
http://i39.tinypic.com/1znq3k0.jpg
http://i41.tinypic.com/o8e7hi.jpg
http://i40.tinypic.com/2gwrhiw.jpg