It's not too small
It's twice the size it used to be so don't complain. It can just as easily be reset to the old size
Also this is in the wrong section :\
Btw the answer to your question is
Upload the image to a webhost
Put the link to the image between [ IMG ] and [ /IMG ]
Removing the space of course
Check the rules regarding sigs in the AUP located in the FAQ