From the WUS rules:
11 Member signatures should contain images with a combined size of no larger than 128px high and 1024px wide.
Please limit any verbiage to 5 lines of text in a reasonable font size/style (as determined by the staff). Note that valedictions such as "Cheers, memberxyz" are not included in the 5 line maximum.
While animated signatures are acceptable, please take care not to annoy others with flashing banners or other images that distract from the discussion.