In your sig you just need to insert in forum code or I believe HTML code works as well to control the size. Each user can set that themselves by how they write it.
This is in your profile but I assumed you knew that already.
Or are you looking to make the default font size smaller? If you are doing that you might want to take a look at the inc_profile.asp do a search for sig I think that should turn up the areas that the signature code is in.
yes, very simply in fact since the signatures use css, you can just change the class it uses for one of your own style classes, take a look in topic.asp for formatStr(Reply_MemberSig) and formatStr(Topic_MemberSig) you will see they are contained in a span tag.