Thanks for the reply. I think the solution you suggested is going to work out just fine. I was a bit worried about the SEO implications of a separate page for each bag size, but hiding the child products from search and the product catalog seems to do the trick.
On a side note (also SEO-related), do you know of an easy way to have the product pages NOT have the /products/ before the page title as part of the URL (other than using a re-write rule server-side)?
Thanks again!