Answer:
As long as the No-Go Zone is fully contained within the mapped area, there is no maximum size limit. As for the minimum size limitations: it totally depends on the turning radius of the mower.
If, for example, you need to create a no-go zone for one tiny little tree, user just needs to manually control the mower during the mapping procedure, to circle around that tree, ensuring that it completes a closed loop, then the no-go area should be created successfully.