Centimeters
Meter is used in finding the width of the blackboard.
To measure the width of bed, the unit meter can be used. It can also be measured in feet and inches.
Identify the SI until you would use the measured the width of your classroom
mm
centimeters
Millimetres (or centimetres to one decimal place).
None! I would measure in centimeters if I were you!
Pencil lead is very narrow. Millimeter can be used to measure it.
Any unit of length can be used. The most convenient number is probably the result of using inches or centimeters.
A tape measure
After you measure the width of the door, you can use any unit of length you like to describe it. No unit is 'correct' or incorrect, but there's always one that produces the most convenient number ... the number that's easiest to write down, remember, and tell other people about. For the width of a door, I think inches would be a bit more convenient than feet.
The width of human hands vary considerably from person to person, female to male. Also, the width will increase if the hand is pressed down flat on a hard surface and then measured. The best thing to do is to measure your hand and use that measurement.