Parts get created in Letter format rather than A4 (the default format for my locale)

What's *supposed* to happen is that parts inherit their style & page layout settings from the "Style for part" (hmm, should probably be plural) you have configured in Preferences, or, if that isn't set, they should get their *style* setting from the parent score but *page layout* settings from the same overall default style that is used for new scores (which is either the hardcoded base style, or whatever you have specified in Preferences as "Style"). For the hardcoded basestyle, the page size is supposed to get the value from the default installed printer - and this should work the same for both scores created from scratch and for parts.

So, do you have either of those Style options set? If so, that is probably controlling your page size.

If not, then do new scores get created at A4? If so, I can't see why parts wouldn't also, but unfortunately, it would be hard for me to debug. Do you have a default printer installed, and is its page size set to A4? I guess it couldn't hurt to post a sample score and steps to reproduce, but I suspect it's more about your configuration than the score or steps.