Can I customize the file layout when exporting via PDF (e.g. image size)?

Yes, you can customize the layout for PDF exports, including the image size, by modifying the Confluence PDF stylesheet. For example, adding the following CSS rule to the PDF stylesheet will ensure that the images in the exported file are displayed at the same size:

.image-container img {
  width: 200px; 
  height: auto; 
}

For more details on customizing PDF exports, you can refer to the official Atlassian documentation: Customize Exports to PDF.