How do I measure the spline for my operator?

Thanks for reaching out to us. Your operator appears to be a Truth brand part, which means your spline size is almost certainly going to be 11/32" in diameter. You should try your best to measure it just in case, though. If you have a pair of calipers, now is the time to use them. You want to measure the total diameter, teeth and all.

As for the best replacement for this operator, I have a couple ideas of what this could be but there are a few important details still obscured. Might it be possible to see this out of the window? I'm chiefly interested in the end of the arm and how it connects to the track/sash. You don't necessarily have to remove the whole operator for that, you can just disconnect the arm and show me that by itself. Either way, it's a detail we can't skip, so that should be your next step.