Are there any plans to support the page size keywords that are defined in the Paged Media spec?

https://www.w3.org/TR/css-page-3/#typedef-page-size-page-size

I need to use "US-Letter" in Prince and "letter" in PagedJS to get the same effect.

Prince already treats both keywords identically.

Of course it does. I'm not sure why I thought that was the issue in my case (it was something else obviously) but I didn't see them listed in the documentation so I just stopped there, thinking that must have been it.

https://www.princexml.com/doc/page-size-keywords/

Oh, thanks for pointing it out - we fixed the documentation!