🚀 go-pugleaf

Use three directories - one for the master FM document, and one "build" directory for each of the formats you need. When it's time to build, copy all the files into the relevant build directory and do your stylesheet applying, book regenerating, and printing in the build directory.

If you decide to handle it in FrameMaker (rather than Acrobat), then use two stylesheets - one with all the formats you need and letter-sized pages, and one with no formats and with B5 sized pages. This way you only need to manage one stylesheet for most formatting controls (except page layouts), and you only need to import the page layouts when you want to build a document or "toggle" it between different page sizes. How similar these two stylesheets are to each other (on the master pages) depends on how much you think you can get away with:

If the page sizes are similar, use different pages sizes but identical text frames on the master pages if you can. This would give you identical pagination although the margins would be different. You might be able to find an optimum "compromise" text frame size that fits both page sizes reasonably well.

If you can't use the same text frame sizes on the master pages, then of course your pagination is going to change, but at least try to use text frames that are the same WIDTH. This avoids needing to manually adjust side-head areas, pictures, and table widths every time you change the page size.