The text you typed is too long. Please shorten it and try to save it again. FIXED

If you are pasting a corporate signature and you are getting this error, it might not be due to the 8kb size restriction.
I was getting this error when accessing OWA and trying to add my signature in Firefox. Another user got this error when adding their signature in Safari.

When adding the same exact signature, the same exact way, in Internet Explorer we were able to add the signature.

So, before you give up and say everyone using OWA will have to have plain text signatures, try adding it through IE.