Hey, Roger. I'm not sure why your window has two different sized springs, actually. They should be identical.
If you can confirm that you have the Series 385
channel balance, then check out the S385-21
. There, you can see a list of all the options and how much weight each option can support. I recommend that you weigh your window sash and see what the weight is, then match that weight to the option that matches. If your weight is different from anything we have listed, let me know and we can try to work it out for you.