Meters or feet would be best, depending on where you live.
liters
A micrometre. A millimetre would be too big a unit but a tenth of a millimetre should do fine.
The volume is best measured in millilitres still. The weight is the best approach for this. To measure size i.e. width in particular, a millimetre is a good unit to use.
You can invent any number of nonstandard units, but none of them is "best". It is best to use standard units, that is, the meter.
You could use cm, meters (eg. 0.18cm ) or millimetres . A mix of cm and mm should be best for accuracy .
A caliper would be the best tool for measuring the width of a pencil lead. It is precise and allows you to measure small dimensions accurately.
Either 'inch' or 'centimeter' would do the job nicely.
Millimetres.
Answer: meters
liters
the metric system
The metric unit best suited to measure the width of a highway is meters. It provides a precise measurement that is commonly used for such infrastructure projects.
You would use the metric unit "meters" to measure the length of a room.
A micrometre. A millimetre would be too big a unit but a tenth of a millimetre should do fine.
A millimetre.
I would suggest putting the camera in front of the door to your bedroom. Putting the camera in the bedroom is not as safe as outside the bedroom. Also, putting the camera in front of the door would probably give you the best chance of seeing an intruders face.
For the diameter of hair the best unit is micrometre.