Answers for "btn extra small bootstrap"

large button in bootstrap

<!-- Large buttons -->
<button type="button" class="btn btn-primary btn-lg">Large button</button>

<!-- Medium Buttons -->
<button type="button" class="btn btn-primary">Large button</button>

<!-- Small Buttons -->
<button type="button" class="btn btn-primary btn-sm">Large button</button>

<!-- Block Buttons -->
<button type="button" class="btn btn-primary btn-lg btn-block">Block level button</button>

btn-default class in bootstrap 4

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

<div class="container m-3">
    <a class="btn btn-outline-secondary">outline-secondary link</a>
    <button type="button" class="btn btn-outline-secondary">outline-secondary 'button'</button>
    <br><br>
    <a class="btn btn-outline-light">outline-light link</a>
    <button type="button" class="btn btn-outline-light">outline-light 'button'</button>
</div>