Double hung tilt window balance replacement

Thanks for posting. If the diameter of the outer tube is 5/8", then you have a Series 600 Spiral Balance. Now it's a bit hard for me to tell the length from just viewing an up-close picture of measuring tape alongside the tip of the balance. Be sure to not include the colored tip in your measurement. However, if the outer tube measures around 28-1/2" long, we recommend that you simply round up to 29" and go with the S600-29. You can afford a little extra length with spiral balances, but you can't get away with less, because then you risk the spiral rod getting detached from the outer tube. Finally, it looks like you will need a blue tip option, which is for a window sash in the weight range of 12 through 31 lbs. 

Measuring the white tube for the tube diameter is best since our balances comes in a metal casing. So, it sounds like you are in the right track on getting the replacement you need. I just want to confirm that when you are measuring for the length, you are measuring the same way as the diagram below. 

How to measure the tube length of the S600 spiral balance.

If everything is a yes, then you should be ok to go.