I found this link that might help.
https://www.lushstories.com/forum/yaf_postsm1976046_Lush-Guide-to-Forum-Signatures.aspx
I hope this helps. I used my Mac Air, so your PC may calc it a bit different. The square is 150x150 pixels and as long as you stay under 75 kb it's fine.
I've numbered the 1-10 vertically so you can see it's just an approximation.

Just my own personal pet peeve, lol! I think I actually put it on another thread a while back...ok, so I'm OCD, but if you are too, this might help
You can highlight and drag the pink box into your signature area to confirm it's size. I find that helps a lot
(Nope! see the following posts...I yai yai yai.....)
