ML RAW resolutions / settings?

[kye]

I'm gradually 'making friends' with ML RAW on my 700D, and this is probably a stupid question, but I can't work out how to select some of the resolutions people talk about.

When I go into RAW video (MLV) --> Resolution I can only choose from 640, 960, 1280, 1600, etc.  People talk a lot about 1344 and other modes but I can't find them.  Changing the aspect ratio only changes the vertical resolution, not horizontal.  I know that if I choose 1920 or above it selects 1728, but what about modes like 1344?

I've tried the nightly build, I've tried the 10/12 bit RAW Video experimental build 700D115 (released Jan 31 2018), I've tried the crop_rec module/build (magiclantern-crop_rec_4k.2018Mar10.700D115) and looked in the 3x3 crop mode and the 720/60fps canon mode but no joy there either.

I've looked on the ML forums and YT videos but can't find any complete instructions.  I'm probably just missing a step somewhere... any help would be greatly appreciated!