The unit of length most suitable for measuring the thickness of a cell phone is millimeter or inches or centimeter.
Explanation:
Cell phones are best used which gets in the size of palms in terms of length and very slim. Mostly the unit of length for the thickness is usually measured in meter scale but as the cell phones tend to slimmer than your palm it generally comes under centimeter scale.
So, it is desirable for the cell phone to be below the centimeter scale. Hence it is specified in terms of centimeter or millimeter. But as technology is enhanced to obtain slimmer smart cell phones, it is most desirable to specify the thickness in terms of millimeters.