bootstrap4.css
.input-group-lg > .form-control:not(textarea),
.input-group-lg > .custom-select {
  height: calc(1.5em + 1rem + 2px);
}
bootstrap4.css
.input-group-lg > .form-control,
.input-group-lg > .custom-select,
.input-group-lg > .input-group-prepend > .input-group-text,
.input-group-lg > .input-group-append > .input-group-text,
.input-group-lg > .input-group-prepend > .btn,
.input-group-lg > .input-group-append > .btn {
  padding: 0.5rem 1rem;
  font-size: 1.25rem;
  line-height: 1.5;
  border-radius: 0.3rem;
}
bootstrap4.css
.input-group-lg > .custom-select,
.input-group-sm > .custom-select {
  padding-right: 1.75rem;
}
■フォーム:form-group:コンポーネント:入力グループのサイズ調整 大、(中/標準)、小 input-group

Small
Default
Large
      <div class="input-group input-group-sm mb-3">
        <div class="input-group-prepend">
          <span class="input-group-text" id="inputGroup-sizing-sm">Small</span>
        </div>
        <input type="text" class="form-control" aria-label="Small" aria-describedby="inputGroup-sizing-sm"> 
      </div>
      
        <!-- 標準サイズの入力グループ -->
      <div class="input-group mb-3">
        <div class="input-group-prepend">
          <span class="input-group-text" id="inputGroup-sizing-default">Default</span>
        </div>
        <input type="text" class="form-control" aria-label="Default" aria-describedby="inputGroup-sizing-default"> 
      </div>
      <!-- 大サイズの入力グループ -->
      <div class="input-group input-group-lg">
        <div class="input-group-prepend">
          <span class="input-group-text" id="inputGroup-sizing-lg">Large</span>
        </div>
        <input type="text" class="form-control" aria-label="Large" aria-describedby="inputGroup-sizing-sm"> 
      </div>