Hi Sophia, there are several ways to do what you ask. Probably the easiest one is configuring the catalog taxonomies to allow selecting more than one (I would say it's "multiple selection" in english). What that means is that you can classify a product inside boys, shirts and medium at once.

Another issue is listing products corresponding to several categories. You can create a menu on the form www.yoursite.com/taxonomy/term/22 (or taxonomy/term/22 is you have activated $base_url) which offers a page listing all the nodes related to the taxonomy term number 22. You'll have to figure which id numbers have each one of the categories you have set up. That's exactly the same that does the catalog blog of ubercart, since it's listing nodes for each categorie. The nice part is that you can make a link to taxonomy/term/22+31+... and now it will show up all the nodes related to all this categories. If you find those url are ugly then use path modules to rewrite the url.

Hope that helps