There are 2 predefined button styles:
And 3 sizes:
Color settings are common for all 3 sizes.
You can specify font family, size, line height and border radius for all buttons sizes individually.
Settings from this page affect buttons in all standard forms (comments, contact form, etc.) and some third-party plugins (WooCommerce, Contact Form 7, etc.).
Note that when you insert button(s) via Visual Composer you can further customize its appearance.