How big is the world in Minecraft Pocket Edition?

This question is quite vague, however I believe that you are asking what size the entire world is in Minecraft pocket edition.

According to Minecraft Wiki:

The Pocket Edition world is 255x255 blocks.

Please note that as of version 0.9.0, the world is infinite.

As of the 0.9.0 update, worlds are infinite. This means that the farther you walk out, the more chunks you load. Unfortunately, there is a "limit" of sorts. This is called the farlands. After traveling out 12,550,745 blocks, you'll be at the very edge of the farlands. It's rather unlikely you'll ever reach that point though, due to these factors.

• Gaps between chunks at about 100,000 blocks out.

• Jitteriness in player movement at about 700,000 blocks out.

• The world crashes regularly and is completely unplayable at about 900,000 blocks out.

If you go truly quite a distance to 32,000,000 blocks out, you'll encounter what Tommasso Checchi has dubbed, the Stripe Lands. Blocks past this point are "fake" and you can fall right through them.

If you choose to make an "old", or limited size world, the world size is x1000 y128 s1000.

Farlands

Stripe lands

The height is set at 64 blocks high.