I think it would be best to have another recordset that filters for the inventory id. If you have the select list set with the inventory id as the value you could have the show if region alternatively set a hidden form element to the inventory value if there are not multiple.
This might be a little tricky to get in place but if you do it this way you should always have a from element on the page that will post an inventory id, either for the selected color or the regular inventory id for the product. If you think this will work and need some help getting it in place you can post back with the page that has the size select list with the show region around it. You will also need to have another recordset on this page that is filtered by that inventory id. I can help you add in the else part for the display to get the hidden form element populated with the inventory id.