The stock design of the Maslow has the best accuracy in the center of the sheet, but the full size of the cutting area is configurable however you would like (meaning with a big enough machine, that best accuracy area could easily cover your full sheet). Also, with work, people have achieved excellent accuracy (millimeter scale) over the whole range of their cutting areas.
You may want to spend some time perusing the forum to see what you are likely to encounter for cutting accuracy and ways to improve it. As for utilizing the whole 1220x2440 sheet, that is dependent on how you build the machine.
You should understand, though, that you will probably go through a sheet or two at least in the initial stages, and mistakes in CNC are all but inevitable (even for professional shops, but they figure those losses in to their costs).
In the end, I think you will have fun if you do it yourself, and depending on how much the quote is, you could save a bundle of money. Not to mention being able to tweak your design and cut it again, and all the other stuff you will find that you never realized you needed to cut!