For ideas, here’s a desktop launcher I created for changing to/from 1.5 and 2.0 scale on the fly: Tutorial: Create Shortcuts to Scale Display Up/Down
It would probably be fairly simple to incorporate the command into a script instead, with a condition such as “if Geary is running, then…” (I’m paraphrasing a potential script element; it wouldn’t actually be written thus.)
P.S. I don’t have any idea if you can set scale in the config file; I don’t have Geary running on my L5.