How do I fix weird page size issue with iA Writer while exporting to Pages?

In order for iA Writer to export their content to Word, they most definitely have "influence" with that exported Word document content and formatting as they had to write the iA Writer export module for that purpose. Their support staff are passing the buck on culpability…

That said, Pages uses its own "black box" Word document converter to translate that exported Word document from iAWriter into Pages own internal .pages format. Just as you have no control over iA Writers export functionality, neither do you with Pages Word translation facility. Apple has never provided any guarantees of completeness or accuracy of that Word document translation process. Nor do they comment on what generation of MS Word the translator is intended to convert.

I suggest that you install the free LibreOffice Suite (choose macOS (Apple Silicon) for the intended operating system) and open the exported iA Writer Word document in LibreOffice Writer. If it looks ok, then simply Save As… a Word 2010 - 365 Document (docx), and then open that in Pages. See if this clears up your current issues with Word documents after Pages translation.

Use the LibreOffice File menu > Save As… to save the repaired Word document as Word 2010 - 365 Document (docx). Then open that document in Pages and see what you have left after the translation.

Nothing quite like LibreOffice to shout out iA Writer corrupted Word documents, eh?

I keep the current Apple Silicon version of LibreOffice installed on my M4 Mac Mini Pro for just these occasions where there might be a misformed Word document that needs repair.

LibreOffice defaults to the OpenDocument standard, so its Writer application will default to the .odt spec on saves. Unfortunately, even though MS Word can open .odt documents, Apple's Pages does not.

If you are in a country that uses the ISO-216 International Paper Size standard, then the default may be A4 instead of the more common North American U.S. Letter size. LibreOffice Preferences has a Language and Locales section that is configured when you install it. That is likely how it determines its default letter paper size.