Need balancers

This appears to be a fairly typical tilt window spiral balance. Assuming so, then you only have two options: either the 3/8" diameter Series 900 or the 9/16" diameter Series 600.

Your next step should be to measure the diameter of the outer tube to verify the better option. Note that you don't want to measure the spring itself, just the outer tube, which in your case appears to be plastic.