If you install Jetpack you'll be able to write custom CSS to be loaded after any other css. I'm sure there are other plugins which will do just the css injection if you don't want the full Jetpack. Or you could add the css to your own theme css.
You'll want to add something like the following in order to change the font size:
It's a bit challenging to say exactly what will work without seeing the css for the btn itself, but the code over should work. Alternatively you can chain the selector with itself in order to increase the specificity:
Don't use !important if you don't need to, you'll just introduce more problems for yourself than you need. That said, if the plugin uses !important to style the button, you'll need to use !important yourself.