How to get grand total filesize of all files matching a filename pattern in the shell?

Try:

find . -name "*.undo" -ls | awk '{total += $7} END {print total}'

On my system the size of the file is the seventh field in the find -ls output. If your find … -ls output is different, adjust.

In this version, using the existing directory information (file size) and the built-in ls feature of find should be efficient, avoiding process creations or file i/o.