Generally speaking, too many characters have nothing to do with the screen size.

Is your signature too descriptive?

Generally, you have your own text description besides pictures, right? If it exceeds a certain number of bytes, a limit will be prompted.

Of course, if you only have a signed picture, it may be that your picture address is too long, exceeding 100 characters, such as:

http:// 12345。 . . . . 100.jpg

This will exceed the limit, and you can use it by uploading your picture to a website with a shorter address.