135: smaller aircraft only, but I have seen Twin Otters operated under 121 (scheduled ops)
It's not about the size or weight of the aircraft....it's the number of available passenger seats. A large long haul jet can be operated under part 135 and flown for hire around the world as long as the passenger seating configuration is under (I believe the numbe is) 20.
There are some other limitations but that's the basics.