a postage scale or nutrition scale
It probably depends on what cell phone you have.
mm
Probably inches or centimeters.
None of them. An ounce (not once) will measure its mass. To get weight you need ounce-weight.
The best unit is a litre.
The best unit for measuring the length of a pencil is inches.
Gram
For a desk like one used in schools the best unit would be pounds. Good luck on your measuring!
centimetres
inch
pounds