Going by the Levi's listed sizing (and yes, these aren't Levi's, but I wear a size 10 in that brand, as they list those as 10/30 lately, and ordered a 30 here which fit fine, so I am assuming equivalence), it looks like a size 6 would be a 28.
Most denim manufacturers state that a pant size 6 compares to a denim size 28. I'm almost always a denim size 27, which compares to a pant size 4. That said, I'm almost always a size 6 in shirts, coats, dresses, etc... It might be best to start with a size 28 and exchange if necessary.