Then use a tailors tape (inchitape) and wrap around the pulleys as they go, remove some for the belt adjuster.
When i was in Pk, i used to make a small laminated card with spark plug and belt length dimensions and zip tie it in the deckie/boot for the owners to buy the right part at first try. (I had to tell the owners what it was so their enthusiastic drivers did not throw it away)
It you cannot measure then tow the car to a shop and let them deal with it. Or ask the ac shop you had it “altered” from.