Np.
Technically speaking, there is no programmed limit, or at least one that would stop images of noticeably large sizes. You could probably have an image as tall as 1000pxl, I'm guessing.
However, it's generally assumed that no one wants to scroll down a large image every time you that you post, so it's forum etiquette to avoid tall sigs like that. Personally, I think 500pxl is a reasonable height limit to set yourself.
That being said, I can't say that a mod won't say something different, but I think their answer would be the same: "As long as it doesn't make scrolling down difficult."