As you have already figured out, until you get to the POS versions of Quickbooks, their idea of managing inventory sucks to put it 'politely'
Since I was moving from an Access Database over to Quickbooks, I created a report that generated all the combinations of style, size and color and exported them from Access into an Excel spreadsheet. Then I used the custom import to bring them all into Quickbooks. I end up with a ton of items in my list but it was easier to do them all up front then to try to add them by hand later.
Now imagine trying to do this where you keep track of things by design, style, color and size