This is the commonly used size: we found no requirement published by the institution. Confirm it before your application.
Tips to get it right the first time
One PDF per proof (identity, address, date of birth) unless the portal asks for a single file.
Protean's guide gives about 300 KB per page: the app compresses the PDF for you.
Make sure the name and date of birth match exactly across all documents.
How to do it with Requisito Listo
Open «PAN application: proof documents».
Scan the pages with the camera or pick a PDF or photos you already have.
Put the pages in order.
The app builds the PDF under the size the portal asks for.
At the end you'll see a checklist: size, file weight, background and format. If something doesn't comply, it tells you what to fix before the portal rejects it.