It's pretty amazing the first time you see them eat a "big" meal. They're amazing animals and easily eat prey item far larger than their heads.
Aside from the feeding chart, some other rules of thumb are: a) feed prey items approximately 10-15% of the snakes weight and b) feed a prey item approx as large around as the snakes widest part. After eating you should be able to notice a subtle lump in their midesection.