Ensure that touch targets are at least 9 mm high by 9 mm wide. (48 x 48 dp)
Separate each target by 8dp of space on all sides
Provide inactive space around the target. The button itself could be smaller than 9mm, but the space around it is also clickable and falls within the 9mm region
Pointer targets are similar to touch targets, but apply to the use of motion-tracking pointer devices such as a mouse or a stylus. Pointer targets should be at least 44 x 44 dp.
Buttons should be easy to reach, regardless of device positioning (e.g., vertical or horizontal)
Strive for flexibility. Do not assume that all users have full range of motion with thumbs, or are right-handed.
Some Mobile OS allow for users to temporarily shift display downwards or sideways to facilitate one-handed operation