How does one determine the appropriate balancer length for a given window?

A handy person from Ashland, OR says:
My window, it turns out, had one 25" and one 26" balancer -- the longer one on the left. They appear to be different manufacturers. I want to replace both. But which length is "correct"??

Our sashes on a pair of windows weigh 27 lbs, each. They have been hard (heavy) to open for the ten years we've had the home. We now need to replace the balancers as one of them has broken. Your chart indicates an S385 with the code 2550 (26").

My question is: should we consider going to a stronger balancer -- to get a nicer 'lift'? The balancers we are replacing are different -- one is a 2640 (left) and the other one is 25^4, where my ^ character means 'space'!

thanks ... Tom
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Tom from SWISCO responded:
Thanks for posting! First, let's focus on the length. What is the height of the visible glass for the sash these balances support?

As for the strength of the spring, it's best to stick with the stamp option that matches your sash weight. Going with a stronger spring won't make the window operate more smoothly; in fact, it would make it harder to close. I think the trouble you're having is likely due to having two different sized balances in the window.
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