Spiral Window Balance Replacement

I need to replace 4 window balances, but I've found sizing them is really difficult. I believe it is 3/8 in. across, but do I measure the interior spring or the plastic covering as shown in the attached picture? Also the length is an issue. Will it be 24 in., the length of the interior spring, or 25 in., the length of the covering plastic sleeve? It's a single hung balance with 2 pins. Also the window itself is about 20lbs. From this description can you recommend a replacement?

And what's difficult about choosing the correct balance is on the old balance the Plastic covering is 25 in. long
the circular spring is 24 in. long. and
the inner most cork screw is 26 in. long.

So which measurement do I us?