I recently came across a radio shack CB/HAM mag mount antenna (#21-972).
I want to trim it down for 2m use. What would be length I would look for?

Hi Mac, Just make it a little long, and trim for best match (swr). You should
be OK with around 19 inches, but if you want the best match at your frequency
of interest, make long and trim.
73 Gary N4AST