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The same size guidelines go for feeding live as any other method. One prey item as thick as the thickest part of your snake or 10-15% of the snakes body weight. You'll want to make sure that you're buying your live prey from somewhere decent. A hungry rodent can put up more of a fight than one thats been taken care of properly and possibly hurt your snake.
You'll want to make sure to supervise the feeding, usually my snakes strike and coil within the first minute I put the mouse in. Anything longer than 10-15 minutes and I remove the mouse and try again next week.