Don't necessarily go by the length of the snake, go by the girth. The widest part should be your gauge for prey/food items. The width of the prey/food = width of the snake. Go here for the care sheet which explains it all...
PS - some go by weight as well, but I found the above to work fine for past snakes and current BP (08)
I am feeding currently adult f/t mice once every 5 days.