Avatars and signatures are removed more often than not because they exceed the size allowed here. If for example a user post a single sentence and the post (because of large signature and avatar) is stretched over the whole screen, then there is a problem.
As such, avatars size limit is 128x128 pixels. I may tolerate a slightly larger avatar as long as it do not exceed the limit by much and have some mercy to our periphery sensorial nerves; if your avatar is animated it must be subtle and not too distracting.
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<font size=2>Good
(subtle and not too distracting):
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<font size=2>Bad
(distracting and repetitive in an annoying way):
</td></tr></table></center>
For the signatures size limit:
<font color=00BB00>Green (580x60 pixels)</font>: Usually this is the typical size of a quote, 3 lines of text (using Mozilla or MIE with a text size set to medium) or a small banner, width is not really important but you should not get over 700 pixel as this will stretch the thread horizontally.
<font color=BB0000>Red (640x120 pixels)</font>: Usually this is around the normal size of an image signature. This is equal to 7 lines of text if you use a text signature.
<font color=0000BB>Blue (700x180 pixels)</font>: Consider this as a maximum. This is the size of a signature with text and image could look like. It is big but it should do fine even if you post often. You must change your signature if it exceeds this size. This is equal to 11 lines of text and you shouldn't use such a huge quote for a text signature.