Due to most 16:9 and/or 3:2 aspect ratio, I frequently want more vertical screen real estate when in portrait mode, but need more horizontal real estate when in portrait mode.   So I typically want:

• in landscape mode:  the start menu & task bar  to auto hide and/or be on the right/left hand side during to preserve vertical screen availablility. 
•  portrait mode: (usually in also in tablet mode for reading) I prefer the taskbar on the bottom.  Moreover since there is ample vertical space but less horizontal space for icons in the taskbar (in portrait mode)  I generally prefer the task bar extra wide/tall to see more icons.

Is there a setting in start 10 for this auto adjustment?

If not that would be a huge usability gain for Windows 10 2-in-1 use cases.