The actual cutting size of the bed is currently approximately 19.47" by 10.97", so it helps to design your files just a little bit smaller than 20" x 12", although it does help to leave the artboard that size. There is a little bit of space lost to deceleration on engraves as well, so if you are engraving, plan on keeping the design scaled to under 18.5" in width.
As far as placement goes, the fisheye effect of the lid camera causes distortion out at the edges of the bed. You can possibly reduce the distortion by running the Lid Camera Calibration discussed here:
That along with the new Set Focus tool do a very good job of helping with placement issues.