Add size 'inherit"

[tresf]

Currently, if a PDF is provided that's 4x6 and it's sent to a printer with 4x6 paper, it will use the printer driver's default page size for printing (e.g. US Letter on macOS)

This is a placeholder to add an option to 'inherit' (or infer) the page size from the document being printed (e.g. a 4x6 PDF will automatically supply size: { width: 4, height: 6 }.