guix size with manifest?

[Spencer Skylar Chan <schan12@terpmail.umd.edu>]

Dear Guix,

How do I run guix size with a manifest? Most guix shell commands use -m 
for a manifest file, but guix size uses -m for map-file. Right now I'm using

grep -F \" manifest.scm | cut -d \" -f 2 | xargs guix size

but this would only work for autogenerated manifests from
guix shell --export-manifest and not manifests written by hand.

Thanks,
Skylar