Disclaimer: I'm doing this question (and answer) based on this:
it is not merely OK to ask and answer your own question, it is explicitly encouraged.
I suggest something like:
find . -regex '.*\.bak' -print0 | du --files0-from=- -ch | tail -1
duare there to avoid issues with whitespace in file names
./dir1/subdir2/file.bak, not just
file.bak, so if you modify it, take that into account
hflag for du to produce a "human-readable" format but if you want to parse the output, you may be better off with
k(always use kilobytes)
tailcommand, you will additionally see the sizes of particular files and directories
Sidenote: a nice GUI tool for finding out who ate your disk space is FileLight. It doesn't do regexes, but is very handy for finding big directories or files clogging your disk.