Just in case any Bootstrap 3 users come across this old question, here’s the BS3 way:

<select class="form-control input-lg"></select>
<select class="form-control"></select>
<select class="form-control input-sm"></select>

<input class="form-control input-lg">
<input class="form-control">
<input class="form-control input-sm">

There is no input-xs, though, so you’d have to make that yourself if you wanted smaller.

.input-xs, select.input-xs {
  height: 20px;
  line-height: 20px;
}