Yeah, 6 gal should be ok for a 5 gal batch, but I'd use a blow-off tube instead of an air-lock. Some of mine pushed the air-lock out, and there are stories of people having beer splattered on their ceiling using 6.5 gal carboys with air-locks.
Also, be careful with glass carboys. I prefer to use glass over plastic, but I have a nasty scar on my wrist from one that I broke
. Not to mention the lack of feeling in my index and middle fingers and my thumb due to a severed nerve. Someone started this thread
about making them safer. Just something to think about...