Since the inventory table has entries for each product you can just have a recordset that selects all from this table and filters it on the product id. If there is more than one record for that product your rs will return more than one record.
You could have a sizes rs that selects from this table and filters on the product id, you could then use this recordset to populate the size select list. If you also filter this recordset on the inventory being greater than 1 you will only show sizes that are available.
You would then have a show if region around the select list based on the recordset. This way if the product has sizes available and there is any inventory of those sizes it will show in the select list, otherwise this list will not appear on the page.
Is this what you are going for? Please post back with any questions or additional info about this.