I can't find a way to reduce the width of the input window. I don't see or find any code that controls the width of the input window. The code below is taken from the Bootstrap v5 documentation, from the form element.
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous" />
<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl+NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP+JcXn/tWtIaxVXM" crossorigin="anonymous"></script>
<form>
<div class="input-group mb-3">
<span class="input-group-text">@</span>
<input type="email" id="email" class="form-control" placeholder="Your email" aria-label="Email" aria-describedby="basic-addon1" />
<button type="submit" class="btn btn-primary btn-md">Sign up</button>
</div>
</form>Because of using form-control, it is taking the full width allotted. You can use the column property like col-md-4 or other property to make the size you wanted.