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.
The number is 30 seats, and the limit is based on the type certificate. A 777 is type certificated above 30 seats, so you could not operate a 777 under part 135. A BBJ is type certificated at 30 seats or less, so you could operate it under part 135.