I just finished putting the lower drive belt on a Troy Belt TB30R I bought a 1/2"X 34" belt and it was too loose. Then I tried a 1/2"X32" belt and it was too tight. A 1/2"X33" belt works just right.
Rural King has sells the belts in graduations of an inch for about $6 each.
It can be a pain the first time, but after doing it three times it makes sense.
By the way, the main pulley on the engine shaft has to be loosened and pulled down low enough to clear the belt guards. There's no way to either get the old belt off or the new one on without removing the nut that holds the pulley on and pulling it down enough for the belt to clear the guards.