Sorry, this is impossible at the moment. But, when MCPE 0.9.0 is released, along with PocketMine 1.4, there will be different sizes: legacy(256 by 256), small, medium, large, and infinite. The legacy world is the one we have now.
I think you mean LifeBoat Infinity. This uses some trickery where once you get to the end of the world you will be teleported to a new world. This makes it seem infinite but actually it's just a bunch of worlds together. But @williamtdr isn't releasing the plugin that allows this.
As @Dinokiller said, that's just a plugin that make you to teleport to the next world once you get at the last block of each world, it "makes" the sense of an infinite server, and it's pretty cool!
