Too big, for me, is when it becomes a bottomless pit and getting to things takes too long. There was a beautiful hobo I got from Nordstrom's last year, and there wasn't even a pocket for a cell phone. I can't imagine trying to find it at the bottom of the bag when it was ringing.
I had to laugh when I saw the title of your thread ... but, to be frank, I'm always looking for the largest sized bag possible. Why? ... because I travel a lot for my job (overseas), and I hate ush: to have to carry both a purse -AND- a briefcase. So, the big bags do the trick for me!!
PS: I'm also 5'10" in stocking feet ... so, big bags don't look ridiculous on me (that always helps too!).
I had the MJ Stella bag but exchanged it for the Sophia because I felt it just looked way out of proportion for my size (I'm 5'2" on a tall day.) I do shy away from small bags, for both practical and asthetic reasons, but there's definitely a point for me where a bag feels too big.