Ultimately it will be an 8cm x 8cm stamp. Ive just finished carving it and will be making the wood mounting later today.
This is how an 8cm x 8cm stamped image would look on a 12.5cm x 22cm brown paper bag (standard stall food wrapper). The brown bag is actually flat, and without a base. You can fold it at the bottom to make the base like I did which will give you a 12.5cm x 13cm bag even after folding it at the top (refer to the photo above).
If your bag has a a thick overlap behind it where the end of the paper meets, your bag wont be a flat and smooth surface at the front. You must make sure you put extra pressure during stamping where it matters at the front to get an even and clear stamped image.