How much does he weigh? Length isn't as important as weight. I would feed a 3 foot 1000gram female a 100-150 gram rat. I would feed a 3 foot 600gram female a 60 - 80 gram rat. I remember your other thread about feeding multiples. If your BP is 1000grams than you would be fine feeding 2 small rats every week.

And just out of curiosity (not that it really matters) but why are you only feeding female rats?