Help with overlay texture size

I am trying to use an overlay to make a scope for my gun mod, but I don't know what the resolution should be. I just really didn't want to try everything until after 1000 tries I finally got it working if someone already has an idea for what that would be. I tried 16x16 on the texture creator and I could barely see it when I zoomed out a bit.

I also wanted to know if there was a better way of getting a scope, as I just started modding last week.