I guess that you were on the wrong forum when you got that information.
According to the belt guide, all 200 series tractors use a 65.2" long belt that is 9/16" in width. That width is unique to Case and Ingersoll tractors. If you choose a 1/2" wide (4L) then it will sit lower in the two pulleys and appear to be too long. If you choose a 5/8 (5L) belt, then it will ride higher in the pulley grooves and appear to be too short. The correct belt is available from our resident Ingersoll dealer, Brian Hildreth. Brian has the belt you need in stock and will mail it to you on Monday.
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Original Equipment belts are made with Kevlar construction. Yes.. you can go to TSC and buy one of their Blue Line belts and get Kevlar construction but you won't get a 9/16" wide belts and you may have to play with the lengths to get the right one.
Since you are new to this site, please let me introduce you to our Tech and FAQ sections. You will find the parts manuals for your deck in the Parts Manual section and Operator Manuals for your deck in the OP manual section. I urge you to review both and then take a good look at the Mule Drive mounting bracket and harness that the deck is suspended from. You also need to measure the outside diameter of the deck pulley that is driven by the belt to make sure someone has not installed the wrong size. Those manuals will help YOU verify that your Mule bracket is correctly assembled. Perhaps you have the tension spring installed incorrectly. You are there, we are not. We can't see whether the spring is attached as it should be or if there are other problems with the assembly. Decks and their harnesses are subject to damage from striking objects and that can affect the belt length as can improper repairs be past owners.
However, on this site we have the information you need, that is available to help you solve the problem and you are free to take photos and post them in this thread for further help. Instructions on how to do that are in the Help and Feedback forum. If you have further questions, please ask them.