How to make twitter bootstrap buttons bigger?

This has changed again in Bootstrap 4.

Now there is only .btn-lg and .btn-sm

https://getbootstrap.com/docs/4.0/components/buttons/#sizes

here are the sizes bootstrap offers right out of the box.

.btn-large
.btn-small
.btn-mini

read more about it here

it's under Button sizes

Edit:

or you could make your own:

.btn-xlarge {
    padding: 18px 28px;
    font-size: 22px; //change this to your desired size
    line-height: normal;
    -webkit-border-radius: 8px;
       -moz-border-radius: 8px;
            border-radius: 8px;
}

Source located here

This has changed in Bootstrap 3.

Now it is .btn-lg, .btn-sm, or .btn-xs

https://getbootstrap.com/docs/3.4/css/