I have the
F-6 (Little Bit Smaller) and I love it. It's just the right size to carry all my stuff; camera with a small zoom or prime attached, external flash, 3 lenses, spare batteries, filters, card wallet, cleaning brush and lens cloth, etc. I think I even have the camera manual tucked away in there too! It's pretty packed for me but if you just need to carry 3 primes I think it would be a nice size with some room for other things like a wallet and sunglasses. If you would like some photos of the interior just PM me. It's not overly cumbersome to carry though at times I do wish I had a bag that was slimmer, but then I usually just throw my camera and a lens in a couple of neoprene bags and toss them in my messenger bag. It's rugged but I sometimes hate those metal latches because they can ocassionally hit my gear if I'm not careful.
I was lucky enough to have a local shop that has most of the Domke bags in stock so I could check out all the sizes. That made my decision 10X easier.