ok, I see the problem, there are 2 issues causing this to not work:
1) The size select list is not part of the add to cart form.
it is outside of the add to cart form tags, so not included as part of the form post when the add to cart button is pressed.
2) in the add to cart form, the element that passes the item ID is named eCartPurpleEssentials_1_ID_Add:
<input type="hidden" name="eCartPurpleEssentials_1_ID_Add" value="<?php echo $row_DetailRS1["ProductID"]; ?>" >
in the Lookup recordset, you have set the run time value of the paramItem variable to a form element named eCart1_1_ID_Add:
the run time value should be changed to: