Get a kitchen scale and weigh her. WalMart has them for $20-25. Tip - get one with a wide platform for stability, some snakes don't like to stay in the bowl/tub to get weighed and you don't want it to fall over if they move.

Once you know how much she weighs you can figure out the appropriate sized feeder for her using the chart below. Figure 10% of her body weight every 5-7 days until she reaches 500 grams, then a small rat once a week.