Maximum area available to cut is currently 19.5"x11" (actually a few hundredths less than that). So you can’t cut a vertical line longer than 11" nor a horizontal line longer than 19.5". You can scale it down in the UI though.
Someday, the cut-able area is promised to be 20.4"x11.5", but today is not that day. And yes, this means that you can’t cut an entire standard 20"x12" Proofgrade sheet in one go. You have to move it on the bed to reach all of it.
I generally cut full sheet in half more or less on a sliding mitre saw to sizes needed for the pieces. That allows use of the entire sheet avoiding loss of the inch or so on the edges that can’t be reached.