How to make a 100x100 world in Minecraft bedrock

Hi all-

I like giving myself little challenges in Minecraft and thought of a survival world that was only 100 x 100 blocks. (up and down to all levels, though)

How might I construct such a world? Preferably, I'd just be restricted to the 100 x 100 block area with the rest of the world intact, but if it were literally a 100 x 100 block column on a super-flat world I suppose that'd work.

Thanks!

Edit: /worldborder for the win!