I believe this is what we bought. The maximum size they offer is 4x12 feet. They have a list of standard sizes, and they can also cut it shorter before shipping it to you. What they can't do are any sort of punchouts, like where you might want to have a light switch or power outlet.

The whiteboard came with sticky goo on the bottom. Peel off a plastic sheet and press to the wall. This probably works if your wall is raw sheetrock, but it unpeeled from our painted wall. Solution? Heavy duty glue. The guys who did this for me went an extra level and rigged up cross-bracing with wood 2x4's to keep it flush while the glue was curing.

And yes, my daughter seriously digs it. We have an eraser and pens with magnets built-in, so they just stick the board when we're not using them.