This is how I rock mine... It was set up under the headlight and nowhere near the fender, but still managed to scuff it up on bumps... I am running about 275 Lbs though so maybe I use more fork travel than most.
I leave two pairs of clear glasses, registration, multi-tool, air pressure gauge, and some ear plugs in all the time. I can even fit a couple other items in it on top of that, like a pack of smokes & my cell phone.
It's not the greatest picture (took it with my Blackberry), but it gives you an idea. I got this bag from a guy off of craigslist for $20. It's an HD bag, and it perfectly hides the dumb speedo setup on the Iron. It's about 6-7" long, 4-5" tall.
I have the HD one 11" I think and it's under the headlight and is fine. It was like $25.00 from the dealer and it looks and as held up nice. The 8" is just too small to hold anything.
Be careful what you put in your bag. I had a couple of wrenches in mine a few years back on my old Heritage, thought there was plenty of travel room but dented the crap out of the top of my fender.