Book size

I’ve been using scrivener with multimarkdown for eons, but I don’t write a book just every year.

So how in the nowadays does one tell scriv/mmd that I’d like to create a 6x9 inch pdf book? There used to be some black magic with XSLT that (as I understand it) has been hidden under a rug someplace.

Hints? I found the file mmd-memoir-layout-8.5x11.tex but it’s not obvious what to change in there to adjust the page size.