Unable to find A6 paper source setting in Create Booklet 2 on Mac

Create Booklet 2 handles the booklet layout, but the actual paper size normally comes from macOS and your printer settings.

Try File > Print, then look for the Paper Size menu and select A6 there. If A6 isn’t listed, choose Manage Custom Sizes and create it as 105 × 148 mm. Apple documents that here: Print on unusual sized paper by creating a custom paper size on Mac - Apple Support

One thing to double-check: if you want the finished booklet pages to be A6, you may actually need to print the imposed booklet on A5 sheets and fold them in half. That depends on how Create Booklet 2 is setting up the pages.

A6 is one of the standard paper sizes offered by Pages in its Documents panel, or Word 2024 for Mac (Layout > Size -> A6). Then you generate the PDF and may need further assistance from the Create Booklet 2 vendor on how you can achieve your goal if possible, though not suitably documented.

Using Coherent's cpdf (free for personal usage) tool from the command line, I can generate either portrait or lanscape A4 documents that are 8-up on a page sourced from a multi-page A6 PDF:

cpdf A6.pdf -impose-xy "4 2" AND -scale-to-fit a4landscape -o A4lx8.pdf
cpdf A6.pdf -impose-xy "4 2" AND -scale-to-fit a4portrait -o A4px8.pdf

Here is the landcape example. You will need a duplex printer for double-sided output.