Email signature size and image rules
There is no single official cap, but a practical rule used by many teams: keep the signature under 600 pixels wide, under about 200 pixels tall if you use a PNG, and avoid large background images. Heavy signatures look broken on phones and can trip spam filters.
Use a solid colour rule rather than a photograph. Put a real phone and email, not a dozen social icons. Do not include tracking pixels. Dark-mode inboxes can invert PNGs with transparent backgrounds, which is why the PNG export uses a white rectangle.