KReportPageSizeInfo pageSizeInfo contains few page sizes. Before we freeze the API how about adding more, at least as many as "PageFormatInfo pageFormatInfo" in calligra.git and LibreOffice has?
Entire QPageSize::PageSizeId list would be overkill indeed.
Related note: if you open LO Writer's page format dialog, there is no custom size property but just size:
It is reset to size based on 'page size type' selection if the type is not custom. That's different to KReport.
Why I like it?
Technically it's not hard to have that. Backward compatibility would not be affected (?).