window won't stay open

I have what appears to be a 3/8"series 900 spiral balance for my windows. The windows are unbranded and over 20yrs old and the business where they were purchased no longer exists. A google search window parts brought me to your site, lucky for me!! I have measured the length of the balances to be 28 1/2" long. You offer the 27" and the 28" models. I am thinking that the 29" model would probably be the one to get as I am thinking it is better to be longer than shorter. If this is the correct replacement please let me know, if not please offer some guidance on the proper replacement. Thank you.
Sincerely and Respectfully,
Brian