Hi,
The current version of black bear is fixed width and cannot be increased simply by altering the css. We have a new version coming that will allow this.
The reason for this is because the slices are 877 pixels wide - you could open these in a graphic application (fireworks, photoshop) and extend them.
I don't mean simply stretch them - they will look ugly if you do, make the canvas size 1000 pixels - cut of the end piece of the slice - move it to the end of your canvas and then cut a section of the repeating area and stretch that out to fill the gap.
If this seems to tricky we should have a new verison out in a week or so.