Well I am not sure how to restrict the actual file size of the sig, but as for the img dimensions, all you need to do is create another BB code for them to use. I just used
and here is the html needed for it
Code:
<img src="http://www.bidforpower.com/forum/admin/cp_logo.gif" height="101" width="455">
You can just change the dimensions to whatever you want, but this will stretch every image used in a sig to that size.