For cutting, the limits are slightly less than 11" x 19.5". It’s safer to keep it to 10.8" x 19.3" so if you use that as a fixed limit, you won’t wind up having your files throw errors.
But…
For engraving at high speeds, your limits are significantly smaller than that on the sides. The head of the laser needs room to decelerate as it moves side to side, and the size you wind up with is based on the speed you’re using - the faster you go, the smaller the engravable area winds up being.
One of the resident experts did some limits testing on it a while back:
Please keep in mind that if you slow it down, you also need to reduce the power, otherwise you will wind up with a charred mess instead of an engrave.