Different kinds of snakes eat differently. Some are nest raiders and eat smaller rodents, but several at a time. Some eat large meals but eat less often. As a general rule, if you can not see a bulge in the thickest part of the body, this is when you move up in size. Please remember that the size of the head has nothing to do with the size of the meal. You want to look at the thickest part of the body.

Lizards are a little different, as a general rule of thumb you can kind of use the space between the eyes as a guide to the size of the insect. Please research your animal though, there are differences in all of them.