I would measure its length in inches, its weight in ounces, and its screen brightness in lumens.
a postage scale or nutrition scale
Grams would be the most suitable unit for measuring the mass of a cell phone.
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
centimetres
inch
pounds
A millimetre.