The rule of thumb is to not offer prey with more girth than the snake.

Feeding large prey may seem like a good way to help your snake grow and bulk-up, but a common mistake people make is measuring their snakes health in relation to their weight. Snakes can become obese, too. If you feed a very large meal (in relation to the snake), give it time to really work it off before feeding again.

For future reference, you may benefit from this food chart.

I would also suggest you invest in a digital scale. It will help you keep track of your snakes general health, as well as the exact weight of the food items he is eating.

There are many good brands of digital scales, but this one seems to be far and away the most recommended: 7001DX Precision Digital Scale. I use one myself. Its also great for baking! Just use a different bowl.