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 .
Either 'inch' or 'centimeter' would do the job nicely.
Millimetres.
Answer: meters
liters
best u nit to measure the length of bedroom = meter
the metric system
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.
Because of the dimensions involved, that would be measured in km. (kilometers).
For the diameter of hair the best unit is micrometre.
The best unit for measuring the width of a textbook would likely be centimeters (cm) or inches (in), as these are commonly used units of measurement for smaller objects like books. These units provide a precise measurement that is easy to understand and compare.