I work on OnePassApps, the team behind app.onepassapps.com. This is our own PDF Organiser.

When you merge PDF files whose pages are different sizes, something has to give: one sheet size for all of them, or one size per page. Our PDF Organiser does the second one. Each page keeps the size it arrived with, and the exported file proves it.

The Paper row names two sizes and stops there

I loaded five one-page PDFs into the PDF Organiser in one browser session: A4 (595.92 x 841.92 pts), US Letter (612 x 792 pts), US Legal (612 x 1008 pts), A5 (420 x 595.92 pts) and a custom 7 x 9 inch sheet (504 x 648 pts). The Source step read "5 of 20 PDFs" and "5 of 300 pages".

The Export step has a Paper row. With those five sheets loaded it reads:

Paper

1 A4 - 1 Letter - 3 other

Two sizes are named, and the count is right: there is one A4 page and one Letter page. Everything else is folded into "other". The Legal, the A5 and the custom sheet are three pages, and the panel calls them "3 other".…