9# balancer. Case is 1.25" wide and 0.5" thick

Thanks for writing in! It appears you've purchased the S161-09 Roller-Tilt assembly. Since the sash weighs 23.5lbs, you will need to add another coil to the assembly to support the weight of the sash. Although, there is no existing 12lb assembly for the Series 161 balances. With that said, you'll need to use the S180-3 Coil Balance along with the 15-086 Tandem Coil Housing. 

Adding this to the Series 161 should give you a total spring strength of 24lbs, which should solve the issue with the window closing on you.

I hope this helps!