i think you made a mistake.
should be "Custom" at size, not "Original", and let me chose my desire size. I've made some short tests with "Original", but my "1280x..." custom size and no changes.
Format : AVI
Format/Info : Audio Video Interleave
File size : 61.5 MiB
Writing application : MEncoder Sherpya-SVN-r34401-4.6.2
Writing library : MPlayer
Video
ID : 0
Format : MPEG-4 Visual
Codec ID : FMP4
Width : 1 920 pixels
Height : 800 pixels
Display aspect ratio : 2.40:1
Frame rate : 23.976 fps
Color space : YUV
Chroma subsampling : 4:2:0
Take a look:
Size should be not Original, but Custom 1280x536.
And where is that MPEG4 settings window from previous version? Trellis, Constant Bitrate, ClosedGOP....