you can feed them up to twice the girth but the recommended size is 1.5 times the thickest part of the body, boids, boas and pythons can go much bigger than that. medium rats should be fine for a 4-6 foot corn, king etc

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snakes owned at some point;
2 garter snakes
albino cali kingsnake
bull snake.
current collection;
royal python
boa constrictor imperitor
colombian rainbow boa