Right now, the avatar size is 150px by 150px, but if you try and link a larger image, it will compact it to fit within that space.

Signatures are a little more complicated as it may depend on your screen. Best advice is to try something and see how it looks, assuming it is rectangular.

If the image doesn't fit properly or breaks the page, try changing it (or a mod will contact you if necessary).

Cheers.