Old fogey question re: export multitrack QT

720p, many short sequences to tweak/re-edit then export for my audio guy. I want to give him a smaller frame size H.264, with just one Roll: the newly tweaked last week's mix.
In master file, I can get H.264 and the single Roll, but cannot change the frame size.
Other choices don't allow Multitrack QT as a choice.
All day I have been doing double exports by exporting again from QT7 at half size.
There must be a one-pass way to do this, and I know you know how! (I'm really spacing on this…)
TIA,
gretta

Create a custom setting in Compressor. Use that for a custom Destnation in FCPX.

Thanks Ben, but that doesn't get me the single role out of FCP. I could disable all other audio with the Timeline Index, but then I will have to reenable them when I want to get back into the sequence.
today I found what seems to be the swiftest solution, and all within FCP: I'm editing these as compound clips, so Duplicated the clip and changed the settings of the Copy to 640x360. I can export that as a Master file with the one role, leaving my original sequence alone. Works great so far.