i haven't (yet) looked at the files you just uploaded
i have taken the productdetail page you originally sent, given the select list a name, moved it to within the add to cart form and got rid of the syntax error on line 45.
you will also need to add a size column to your cart object and make it unique, then it should work. (fingers crossed)
if you still have problems, let me know and i will check out your files.
if you are looking to learn some php and general dreamweaver stuff, I can recommend 2 books by David Powers,
PHP Solutions: Dynamic Web Design Made Easy
The Essential Guide to Dreamweaver CS4 with CSS, Ajax, and PHP.