the 717 is not an RJ. It's a full size aircraft, but not as large as the 737. If you visit the AirTran site you can learn more about it. The bins are large enough for a reasonable carry-on, and if you can't get it overhead, you can gate-check it for free.
Seating is 2-3.
Perfectly comfortable plane for a short trip. Minimum Biz upgrade is now $69, and I think that is steep unless you are checking a couple of bags and will drink your share, plus the more comfortable seat, priority boarding, etc. No meals, just nicer bags of chips and cookies, vs. pretzels. I recommend checking-in online at T-24 to get the best possible seat. There is an advantage at BWI to being able to jump a sometimes very long security queue.