[applecrazy]

You can adjust the canvas size. There's a place where you can enter the voxel x voxel dimensions you want to work in.

Edit: in a recent update it added support for 1024 x 1024 scenes.

[Flenser]

For those like me that couldn't work out how to create bigger scenes, the world editor is available by pressing TAB or clicking the TAB icon above the top-right corner of the main view.