bootstrap中如何设置表单的宽高

<form role="form">
  <div class="form-group">
    <input class="form-control input-lg" type="text" placeholder=".input-lg">
  </div>
  <div class="form-group">
    <input class="form-control" type="text" placeholder="默认输入">
  </div>
  <div class="form-group">
    <input class="form-control input-sm" type="text" placeholder=".input-sm">
  </div>
  <div class="form-group"></div>
  <div class="form-group">
    <select class="form-control input-lg">
      <option value="">.input-lg</option>
    </select>
  </div>
  <div class="form-group">
    <select class="form-control">
      <option value="">默认选择</option>
    </select>
  </div>
  <div class="form-group">
    <select class="form-control input-sm">
      <option value="">.input-sm</option>
    </select>
  </div>
  <div class="row">
    <div class="col-lg-2">
      <input type="text" class="form-control" placeholder=".col-lg-2">
    </div>
    <div class="col-lg-3">
      <input type="text" class="form-control" placeholder=".col-lg-3">
    </div>
    <div class="col-lg-4">
      <input type="text" class="form-control" placeholder=".col-lg-4">
    </div>
  </div></form>