W3.CSS allows you to easily create buttons of different sizes by using additional size classes.
Here are some common button sizes:
<button class="w3-button w3-blue w3-small">Small Button</button>
<button class="w3-button w3-green">Default Button</button>
<button class="w3-button w3-red w3-large">Large Button</button>
To create a full-width button, add the w3-block class:
<button class="w3-button w3-purple w3-block">Full-Width Button</button>