34mm Mikuni BS series factory. Any 34mm BS series should work. This is a good carb, if the carb is just bad, try rebuilding it instead, much cheaper and pretty easy task. If you are looking for more performance a more open exhaust and a freeflow intake like a k&n, with bigger jetting.
The average carb size for the Suzuki GS 850 is 36 milimeters. If you are unsure you need to measure at the inside of the carb at the venturi in milimeters, this will give you the size. But the typical carb for these older models is the 36 milimeter version.