col-sm-3 col-form-label col-form-label-sm

  <form>
    <div class="form-group row">
      <!-- 小サイズのラベル -->
      <label for="labelSm" class="col-sm-3 col-form-label col-form-label-sm">小サイズ：col-form-label-sm</label>
      <div class="col-sm-9">
        <input type="email" class="form-control form-control-sm" id="labelSm" placeholder="form-control-sm">
      </div>
    </div>
    <div class="form-group row">
      <!-- 標準サイズのラベル -->
      <label for="labelDefault" class="col-sm-3 col-form-label">標準サイズ</label>
      <div class="col-sm-9">
        <input type="email" class="form-control" id="labelDefault" placeholder="Default size">
      </div>
    </div>
    <div class="form-group row">
      <!-- 大サイズのラベル -->
      <label for="labelLg" class="col-sm-3 col-form-label col-form-label-lg">大サイズ：col-form-label-lg</label>
      <div class="col-sm-9">
        <input type="email" class="form-control form-control-lg" id="labelLg" placeholder="form-control-lg">
      </div>
    </div>
  </form>