An administrator asked
"We're using Exchange 2010. We can’t create signatures more than a couple of lines in OWA. Any longer and we get a message: “The text you typed is too long. Please shorten it and try to save it again.” I remember there was a Registry change for Exchange 2003 that would set the max size limit, but I can’t find a setting for 2010."
The method you remembered using for Exchange 2003 won't work with Exchange 2007 or 2010, however in case anyone reading this wants to know how it's done, the administrator set it by editing the registry key at HKLM\SYSTEM\CurrentControlSet\Services\MSExchangeWeb\OWA
The default SignatureMaxLength value is 4096 (4KB) and the maximum value is 16672 (16KB).
In Exchange 2007 and 2010, the maximum signature size is 8KB and it cannot be changed. Note that the 8KB includes the html and the text part, so you need to factor in HTML formatting.
If you are sure the signature is not this large, or if it can be edited on other computers, it could be an issue with your browser. Others who received this error message were able to edit the signature using Firefox or Chrome.
If the signature really is too large, you'll need to use transport rules or a third party utility to add a disclaimer on outgoing messages.