If your theme doesn’t already have a functions.php, you can create one and add that code to it – it will automatically be found and run by bbPress, and it’s a lot safer than adding it to the core code (next time you upgrade, it will overwrite).
1. bbPress doesn’t have signatures by default, so I’m guessing you’re using a plugin for this? If you are, hopefully the designer has added a filter which you can hook into, but we’d have to take a look at the specific plugin you’re using.
2. You should have a style.css file in your bb-templates/yourthemename/ directory. Find that file, and do a search for ‘textarea’. It should possibly be under ‘.postform textarea’ to be more specific. Look and see if there’s a font-size attribute there, and up the size.