balance for NuPrime tilt double hung windows

I have several NuPrime double hung tilt windows are very hard to open and close. A few of them the spiral part won't stay in the tube of the balance. The one balance I was able to take out of the window casing has red tip and is 25 inches long and the diameter is half inch. I couldn't find any marking on the balance. The sash is too heavy that I was not able to take it out and weight it. The sash measured 35 x 25.5. Some of the window actually has two balances within the same side of the window, and some has just one. Do you at least carry the 25" red tip that I can replace mine old one? Do I need to use two balances at each side of the window?
Thank you for your help.

Thank you for your response. I measured the diameter again. The outer diameter is about 9/16 as shown on the enclosed picture. Do you carry a sash stopper as shown in the enclosed pictures? It is 4" long and 1 1/4" wide. Thanks.