series 140 constant force balance

I am afraid we can't estimate weight from dimensions. The best thing to do would be to weigh the sash on a scale. Give us that exact weight, and we can work out what you need. To be safe, I'd also like to see the coil balance systems you're working with, preferably out of the window. Many thanks. 

That's perfect Dave, thanks. This is the 15-175 Pivot Shoe, which uses our Series 152 Coil Balances sold separately. if the window weighs 14.4 pounds, you want to use a S152-7 7 pound spring on each side of the sash. This equals 14 pounds of overall support, which is certainly close enough to the reported sash weight. Let me know if you have further questions.