I'm currently building a town in Minecraft Pocket Edition and I'm wondering how many blocks long and how many blocks wide the world is.
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.
The Old world type is very small and contains just a few biomes, while the Infinite world type, like its name says, is somehow infinite, because of the Far Lands still existing. According to the wiki, the Far Lands have the classic format in the north and just an ocean in the south. I am sure that the Infinite world is infinite from north to south,but I am not sure if it's the same from west to east. The height limit is just below the clouds.