Good idea for a plugin.
Sorry, though, I’m not sure offhand what the best CSS selector is. It might depend on your theme—e.g., some themes have variables for this value, such as --editor-font-size.
Most themes are well-documented, though. You can go into your theme’s CSS by opening Preferences → Appearance → clicking on the folder icon above the theme selection menu, then opening the .css file(s) you find there. In that file, what you’re looking for will probably be clearly labeled.
Hope this helps!