So the depot of the seat can be just about anything, but the depth of the arms (and thus the support for the seat) are maxed out at half the overall width. So that is 16", which is about right.
And I botched the back posts! They are rectangular, and I had planned them with the short side facing front, but cut mortises as if it was to face wide side front.
As a result, better wood for the posts,. Here is the back, assembled. Next will be the arm frames..
And the parts thereof