No brush size keyboard shortcut option

The keyboard on my Macbook pro is smaller than a standard keyboard and doesn't have the bracket keys to change the size of my brush in Photoshop. I've tried going in keyboard settings and then tools to assign a different keyboard shortcut for brush size, but it appears I don't even have the option to do that. The brush size keyboard shortcut does not even appear in the list of possible shortcuts. Is there a way to fix this because using the keyboard is my preferred method of changing my brush size.