change font size bootstrap button code example
Example: bootstrap 4 button size
<button type="button" class="btn btn-primary btn-lg">Large button</button>
<button type="button" class="btn btn-secondary btn-sm">Small button</button>
<button type="button" class="btn btn-primary btn-lg">Large button</button>
<button type="button" class="btn btn-secondary btn-sm">Small button</button>