<button class="btn btn-default" type="button">Default</button>
<button class="btn btn-primary" type="button">Primary</button>
<button class="btn btn-success" type="button">Success</button>
<button class="btn btn-info" type="button">Info</button>
<button class="btn btn-warning" type="button">Warning</button>
<button class="btn btn-danger" type="button">Danger</button>

<a class="btn btn-default" href="#" role="button">Link</a>
<button class="btn btn-default" type="submit">Button</button>
<input class="btn btn-default" type="button" value="Input">
<input class="btn btn-default" type="submit" value="Submit">

The class for size button bootstrap is :

.btn-lg
.btn-md
.btn-sm
.btn-xs

For example :

<button type="button" class="btn btn-primary btn-lg">Large</button>
<button type="button" class="btn btn-primary btn-md">Medium</button>
<button type="button" class="btn btn-primary btn-sm">Small</button>
<button type="button" class="btn btn-primary btn-xs">XSmall</button>