The smallest here at TRI is either the B1900 flying under the US Airways Express banner for Commutair out of IAD, or Bombardier DHC 8's flown by Piedmont Airlines (again US Airways Express) out of CLT. I'm not sure which one is smaller, I haven't really seen them both together here.
We used to get J-31s from BNA (once again US Airways Express), and a few years ago when AA had a hub at BNA we had J-31 service from American Eagle. The J-31s were very small and kind of annoying in a way, like little gnats or mosquitoes...US Airways Express dropped that route I believe, just a few weeks ago though...I haven't seen it on the schedule :-(
As for cargo the smallest we get little FedEx feeders...mainly the Cessna Caravan.
I think these are the smallest as of now, but I'm not out at TRI all the time so I could miss something. But the B1900 and DHC8 are the smallest commuters here. We also get Dornier 328 service, but these aircraft are rather large compared to the two others I mentioned...
"my soul is in the sky" - shakespeare