<select name="size" id="size" >
<option value="41">41</option>
<option value="42">42</option>
<option value="43">43</option>
<option value="44">44</option>
<option value="45">45</option>
</select>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
$('#size').on('change', function() {
alert($('#size').val());
});
</script>