I posted some photos of my F-3XB alongside an F-2B that I recently picked up. You can think of the F-6 as a shrunken-down F2 without the side pockets (a little bit shorter and narrower).
tow . com/2008/12/29/an-ode-to-the-domke-f-3xb/
In the post are links to other reviews of Domke bags.
Note: I haven't posted 10 messages yet, so I can't include actual links in my posts. Take out the spaces in the URL above to make it work.