Route a long piece of string as the belt would be routed. Pull it taught, then tie it off. Cut the string and measure it.
Most brands of belts are supplied in sleeves that explain the relation between belt size in length. Consider that you need a belt one size SHORTER than your string, as the tensioner was not loaded when you tied the knot.
I just measured my car with out the ac comp. It was 82 1/2 inches. I bought a belt size 81.75" from a AutoZone. Works perfect. Just no AC! Had to remove the water holder for the Radiator to install Belt.