There is an end, but you'll never find it. The max size is a bit more than eight times the surface area of the Earth. In all practicality, maps are infinite. --- Roy will always be Our Boy! SoulSilver > HeartGold. Next case.
So you can walk around the Earth 8 times (probably more if you don't walk on every centimeter, but less if you are not gonna swim) before getting bored with Minecraft! --- By the Nine! Stop right there criminal scum!