Determining paper size of PDF via command-line tools?

[Jim Long]

I have been lax in enforcing a paper size in my engraved files.
This has been invisible to me, since apparently XPDF prints A4
files on a US letter-sized printer without complaint.

But some folks that I send PDFs to have noticed (and indeed I've
noticed it sometimes when I print in an unfamiliar environment).

Rather than re-engrave *everything*, I'd like to use a
grep/awk/whatever command pipe to determine the paper size of
certain PDF files.  I'll then re-engrave only those which are
identified as A4.  Is this possible, and if so, can anyone please
provide me a suitable UNIX command line that can identify the
paper size referenced by a given PDF filename?

If not, I can work around this other ways, but I thought I'd ask.

Thank you!

Jim