The value should be added to the value attribute:
<tr>
<th>Shirt Size:</th>
<td>
<input type="radio" name="shirtsize_1" id="shirtsize_1" value="S" /> S
<input type="radio" name="shirtsize_1" id="shirtsize_1" value="M" /> M
<input type="radio" name="shirtsize_1" id="shirtsize_1" value="L" /> L
<input type="radio" name="shirtsize_1" id="shirtsize_1" value="XL" /> XL
</td>
</tr>