Hello GrinGEO,
You can choose any value for the texture atlas size, not necessarily to use a degree of two, so 8000, 4321 and etc. would work.
As for the difference between Orthophoto and Adaptive Orthophoto mapping options, in the first case the texture atlas will contain only single block (in general case) of the orthomosaic that would be applied to the horizontal surfaces only, so all the vertical surfaces would be untextured (for example on the building walls the texture will be stretched).
The Adaptive orthophoto method will create separate charts for vertical and horizontal surfaces, so this option is better for urban areas, it is it necessary to build the texture for the walls as well.