Take a good look at the size of your snake. Don't feed it anything much larger than the widest part of the snakes body. After feeding you may notice a small lump in the snake's body from the food item. This is a good feeding standard to stick by as your snake grows.
As Matt already mentioned, a Hopper mouse every 5 days is fine for a few feedings and them bump it up in size. We've got some 3.5 month old boas that were just bumped up to fuzzy rats and took them like champs. Once we switch them to rats, the schedule is extended to 7 day feedings.
I don't know the age of our Suri (my guess is about 8-9 months); she's about 34" and 250g (growing pretty darn slow =P) and is on rat pups every 7 days.