.dropdown-content li>a, .dropdown-content li>span {
   font-size: .8rem;
}
.select-wrapper input.select-dropdown {
    font-size: 0.9em;
    color: #495057;
}


// HTML

<div class="row">
    <div class="col-md-4">
        <div class="md-form form-sm">
            <i class="fa fa-lock prefix"></i>
            <input type="text" id="formInputFrmAddEvent3" name="date_debut" class="form-control form-control-sm">
            <label for="formInputFrmAddEvent3">Date</label>
        </div>
    </div>
    <div class="col-md-4">
        <select class="mdb-select mt-1">
            <option value="1">Option 1</option>
            <option value="2">Option 2</option>
            <option value="3">Option 3</option>
        </select>
    </div>
</div>