The general rule is to feed 10%-15% of your snakes body weight or a prey item that is equal or slightly larger than the thickest part of your snake.

Grab a cheap digital scale from Walmart and the rest is history.

And yes, a snake will take down a prey item that is to large for them. Problems can occur during the digestion stages if this is the case. Best to stick to the general rule stated above if you are in doubt of feeding sizes.