set the size column to be updatable from a select list, the use javascript to change the value of the size form element, an example of this would look like:
<form name="SoftDipesCart1_4_ATC_1" method="POST" action="/product.php">
<img src="images/thumbs/fishpopup.jpg" align="left" /><br />
Velcro Dipes<br clear="all" />
<input type="hidden" name="SoftDipesCart1_4_ID_Add" value="1" >
Price:
<select name="SoftDipesCart1_4_Price_Add" onChange="document.SoftDipesCart1_4_ATC_1.SoftDipesCart1_4_Size_Add.value = document.SoftDipesCart1_4_ATC_1.SoftDipesCart1_4_Price_Add[SoftDipesCart1_4_Price_Add.selectedIndex].text"><br clear="all" />
<option value="3.00" selected="selected">New Born</option>
<option value="3.50">Small</option>
<option value="4.00">Medium</option>
<option value="4.50">Large</option>
</select>
Name:
<input type="hidden" name="SoftDipesCart1_4_Size_Add" value="New Born">
<br />
<input type="text" name="SoftDipesCart1_4_Quantity_Add" value="1" size="4" />
<input type="image" src="WA_eCart/Images/Pacifica/Btn5_EN_addtocart.gif" border="0" class="eC_ImageButton" value="Add to Cart" name="SoftDipesCart1_4_ATC">
</form>