The Atlanta Hermes boutique is not very big but it would still be a worthwhile trip because Lenox Square mall is awesome. From the airport I would say it would be a 45-60 minute drive, depending upon the traffic which can be horrid in Atlanta. It's been 8-9 months or so since I was there so I'm not able to comment on the current stock. However, the last time I was there I saw both a Birkin and a Kelly on display.
ETA: The SA that helped me the last time I was in there was very helpful. I wish I could remember her name!
Yes, it's true, the Atlanta Hermes certainly isn't a huge store (the new one coming next fall will be three times the size), so of course, there isn't a ton of room for display. However, I wouldn't call it tiny either. But, if you're looking for anything in particular, be sure to ask, as they have drawers and drawers of stock that isn't out.
Driving from the airport would probably, indeed, be a nightmare for anyone unfamiliar with Atlanta (I've lived here for over twenty years and won't even make that drive!). You could store your bags at the airport and jump on MARTA, our rapid transit system. You'd get off at the Financial Center stop and walk a bit north to Lenox Square. You'd see Neiman-Marcus, and Hermes is just outside of their mall door. Or you could also get off at the Lenox Square stop and the mall will be just across the street and slightly behind you. The trip one way would take you about 45-50 minutes depending on the time of day, and the cost is around $2.00. (Just make sure that you're on the Doraville bound train leaving the airport or change trains at Lindbergh, as the route splits there, and you'd need to be on the right train, or you'd never see Hermes!)
The JW Marriott would be a great choice, as it's actually connected to Lenox Square, which means that you wouldn't even have to leave the comfort of the air conditioning. There's also the Intercontinental south of the mall but definitely within walking distance. The price would be higher. If you really want to splurge, there's the Ritz Carlton next door to Phipps Plaza, another great place to be parted from your cash. It's just across the street from Lenox Square.
It's a very hospitable store, and all of the SA's are friendly and happy to help.