If you are talking about image files remotely hosted and being shown as sigs, you can't do it because you have no control over how big the file size is on a remote server.
However, you can install a hack that makes the user uploads the sig file to you here (hack by dabean):
https://vborg.vbsupport.ru/showthrea...threadid=28520
This way you can control the size of the image files since it's on your server.