We don't have a set maximum because, as pointed out above, everyone has a different idea of what's an acceptable size. And, if we say a number, people will panic and assume it's a hard-set limit, when it's just a guideline. For example, if we say that our guideline is 250px, people will panic if someone has a sig image that's 251px.
And, even if we did, we couldn't enforce it using the character limit, because the displayed size of a signature is in no way related to the number of characters in it. For example, my signature is almost a thousand characters, yet it's only seven lines tall. On the other side of the coin, it only takes about 20-30 characters to insert a Giant Image of Doom.
We're currently in the process of discussing this, and any changes to current policies will be posted in this thread.
