Why "letter" (and not A4) is the default page size for instrument parts?

[FredGUnn]

@Lillie_Harris

Have you saved settings in Layout Options as default yourself at any point? For me, Dorico does indeed use A4 for parts by default, as confirmed by resetting layout options for a part layout back to factory (number 10).

And/or where are you based? I think Dorico defaults to Letter when used in the US, whereas I’m based in the UK and have A4.

There is a bug with this though which Daniel described in this post. In that post also states that this will be fixed in the next update. Perhaps this is the issue the OP is encountering with Dorico defaulting to Letter. (Or maybe Kostas has it all worked out now)