I'm not sure what size of signature you are looking to send but you could try making a text file and setting OE to use that rather than the text typed in the text box.

I can't work out the full deatails of how the autocomplete URLs are stored but the system uses the History folder for most of it (It could be all but emptying my history folder still leaves me with some content - seems to be stuff starting without http - in the autocomplete box).

Jon