I don't know which size to order , to replace this ,window balance

Welcome! The 26-3 stamp there means a lot to us. It tells us that your balance measures 27" from metal to metal (which corresponds to a stock 27" balance) and that the stamp option you need is "2630." What it doesn't tell us is the Series. You have either a Series 380 or Series 385. They can be hard to tell apart, but here's the key: measure the bottom shoe. The photo below shows how to do that. If your shoe measures 1-3/32" you need an S385-27. If it measures 1", choose an S380-27.