No doubt, if you are into carpentry however you would call it a "header" rather than a plank :-) The more salient bit though is that the header has to be able to support the weight that the missing wall underneath it would have supported, had that wall been there. Since we're building with concrete (in this hypothetical example) you're looking at a lot of weight. That puts you in the 4 x 12 category of header, maybe two if you are doing a longer space. But that has an issue that concrete doesn't adhere to wood (which is why they make forms out of it) and so now you're looking at a more engineered beam that you can have concrete form around. Except that isn't really very strong unless it is prestressed (which is what they do with the concrete under bridges and roadways, and for that you really want to pull some iron through it which you release after it solidifies (the iron trys to contract and it keeps pressure (aka stress) on the concrete). So now you are 3D printing part of the building, then pausing so that your ironworkers can set up some rebar for the door and window headers, and then you put that concrete in and they come back and trim those. (this looks better than the threaded rod through the concrete with nuts on both sides to add the compression.

My point is not that the problems are not addressable, they are, rather its that "3D Printing" and "Homes" isn't a good fit yet given what we know. This neither invalidates 3D printing as a pretty revolutionary way to manufacture, nor the notion that there might be better ways to build homes, just that this particular thought experiment is a pre-mature at best, and potentially forever impractical.