Don’t delete the prices field in your products table just yet, leave it until you don’t refer to it anymore !
Here's what to do
In your add to cart form on the products detail page, add a select list, with your sizes table as your data source, filtered by productid, so that only the sizes you have associated with this productid will be shown. Let’s call it ‘sizelist’. Use sizeid as the value and sizedescription as the display.
Load the page and confirm it works as expected.
Add your price lookup recordset to your page, let’s call it rsPriceLookup
Pseudocode for this is…
SELECT (fields) from pricestable WHERE productid = url variable and sizeid = post variable.
( Post variable is $_POST["sizelist"])
This should return 1 row with you price.
In your add to cart code, you set your price line to
$ATC_itemPrice = floatval("". $row_rsPriceLookup["Price"]."");// column binding