Hi Nola,
There is a simple workaround. Use Adobe Illustrator to open the PDF. It is read as a vector graphic, which means you can then scale it to the size you want without loosing any quality (no need to make them so big in Agisoft Metashape). You can then also adjust the paper size.
I assume it also works with Inkscape which is a free and open source alternative to Adobe Illustrator.
All the best.
Regards,
SAV