What I do to make sure that I am feeding the correct size of prey is to use a scale to track my snakes weights. I take the scale to the pet store and weigh the rats to make sure they are 10%-15% of the snakes body weight. You can get a digital food scale from Target or Wal-Mart in the kitchen section. Some people use digital postal scales. Some tobacco shops sell them. Or you can get them from here:

http://www.reptilebasics.com/digital-scales/

Hope this helps...