Stef means the height of the image max 100 px.
And I agree with that, 100 is fine.
But do not try to make a signature which has a 100px hight, and 5000 width
I guess you should try to stay within 100x500px.
A little bit larger isn't a problem, but with a signature off 200x400 you have for example 2 lines text in a post, without smileys.
The text is ~16px per line, so 2 lines makes 32px height, for the text.
This means that your signature is 6.25 times bigger. I think thats a little bit too much
And yes, I know that a lot of gamesites allow you to use 500x500 signatures. And they all suck.
You can't read the normal text anymore because all picture spam.