I do 4x5 cyanotypes, I love the size. 6x9 is pretty close. I'd start with the native size just to avoid problems. Get the process down.
I use 4x6 strathmore drawing for cyanotypes, I buy the little books. No trimming of paper needed, 6x9 would fit sweetly on a 4x5 piece of paper.
They make great postcards. Also framing them in inexpensive frames and hanging them in a grid looks very impressive.
Making enlarged negatives can be done but interpositives are a huge pain in my opinion.
I too would strongly consider development by reversal.
You would expose your ortho lith film like paper, process it so it comes out like a b&w slide and you're good to go (once you get the process down)
I've made 3 or 4 enlarged negatives over the years that came out 'good'. I used ortho film, I used panchromatic film for the diapositive, etc. I followed guides. I just suck at it.
If you look at the articles section here on apug there are a large assortment of formulae for reversal processing kindly provided by one of the members. Alessandro Serrao has a great article in the alt. process section, and Ian Grant has provided many formulae in the Film Developer - Non Staining section.
Also, Ilford has a very easy to follow PDF on reversal processing. And 8/16mm reversal processing of movie film is a good, informative read too.