it is 768 x 375.
From the rules:
"8. Signatures
We encourage forum members to exercise positive self-expression. Forum members may include signatures in their posts.
Signatures must not include more than three lines of text.
Signature images must
not: (i) be larger than 500 pixels wide by 100 pixels tall and must not contain animations; and (ii) exceed a file size of 100 kb.
Signatures and signature images that are not consistent with these Rules of Conduct may be removed."
The reason is they start overpowering the content of the posts.
If you down size it a bit and chop some off the top and bottom it would conform.
The picture I have below is 200 x 100.