Big Papy, You are safe with something like 50 x 50, but I see other forums list things as big as 128 x 128. Max size may be 50 kb. This forum should tell you the actual maximum size if you can find the general instructions.
Not to create chaos, but there is no photo size limit for photos that you include in your signature. It becomes an avatar in that it appears in every message you write and its of a size that you are happy with. So if you wish, go into your profile and try your normal sized photo there. My message should show you what I did in mine.
Try this link for a more complete discussion. The bottom line may be that you need to resize you photo on your computer and before you upload it to Photoshop or other service. https://tw200forum.com/index.php?/top...__1#entry12490