On the Category admin page you can create as many vocabularies as you want. Ubercart already creates one, a very basic one indeed: Catalog. Later you can create as many as you need. And look, editing the vocabulary you force drupal to show them every time you edit a node of the types you select here. Every vocabulary you create, edit and select "product" nodes will show this vocabulary when you edit the node, under the body textarea and format.

You can have as many vocabularies as you want related to one concrete node type. Even you'll see inside the vocabulary editing that this concrete vocabulary may be mandatory or not, it means you'll be forced to select an item on the list when editing the (product) node or keep it without tagging. And even more, inside the vocabulary editing you'll see that you may allow to select one term or several, which means that you'll be able when editing the (product) node to select only one entry on the terms list or several.

Now, if you're catalog includes categories boys and girls under gender, and categories S, M, L, XL under size, if you allow on your vocabulary catalog to be able to select several terms for a single (product) node, you'll then be able to edit a product for boy and XL, and two entries on the table that registers that data will be added. Thus you´ll be able later to list all products on taxonomy/term/number-for-boys, by taxonomy/term/number-for-XL or a combination of both as I stated on some previous message.

Maybe it's a long explanation, I hope it makes sense