Cindy,
If you sign the size that you want the finished signature, scan it, and then make it transparent, it will remain the size you want.
With that in mind, I wouldn't tinker with the size if the image field once you start the process of making it transparent, rather, just do your thing, then save the finished one.
I will add that it helps to makes saves along the way so that it easier to overcome individual goofs.
