The scanned objects need to fit onto the turn table and the turn table has to be completely visible to the phone camera you are using.
The height of the objects you can scan depends on your phone camera and how far you place it from the turn table. As a rule of thumb it is slightly higher than the diameter of your turn table. You can increase the supported height by moving your phone further away from the turn table at the cost of losing some scanning resolution.
The supported scan object dimensions are:
ASCAND Standard: Diameter 200mm, Height 210mm
ASCAND MAX30: Diameter 300mm, Height 315mm
ASCAND MAX40: Diameter 400mm, Height 420mm
ASCAND MAX60: Diameter 600mm, Height 630mm