so the including (main) file sets b, and the included (part) file should set its page size according to it? You could probably do that like this:
#show: it => context {
set page ("a5") if b()
it
}
Strangely, it seems that the set page only takes effect if there is some content on the page before. This is probably related to the workaround I had to use here: Why is the page counter in the background of my page wrong after using counter.update()? - #11 by SillyFreak – if I have the time, I’ll see if the situation has changed since the 0.11.1 release. (EDIT: I just tested this using a nightly Tinymist based on Typst 8c813cb (2024-09-22) and it seems to work as intended)
That said, context that goes over large ranges, e.g. entire sections, is indeed not ideal (as far as I know). If the boolean is set once and then just exists (i.e. you don’t update it, it’s just configuration), I would actually use import: Why can't I use a function in a chapter even though I imported it at the start of my main file? - #2 by SillyFreak
It should be possible to import "main.typ": b if you don’t want to put the variables in a separate preamble.typ (or similar) file.