The new --format parameter would allow for values like:

• Letter
• a4-portrait
• A4-landscape
• 210x297

All the pages in the output PDF would then have this format.
Images provided to pdfly x2pdf would then be fitted to the page.

[EDIT] More specifically, we would like to support all valid formats given as $commonNameForDimensions or $commonNameForDimensions-$orientation.

$orientation is optional, can be landscape or portrait, an defaults to portrait.

$commonNameForDimensions could be any Ax standard format: https://en.wikipedia.org/wiki/Paper_size#Overview_of_ISO_paper_sizes (with lowercase or uppercase first letter). It would be nice to also support Bx & Cx page formats.

Finally, Letter refers to the US Letter format: https://en.wikipedia.org/wiki/Letter_(paper_size)
We should also support Legal for US Legal: https://en.wikipedia.org/wiki/Paper_size#Loose_sizes