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.
20 inch
liters
metres would be the best way to work out the length of a classroom as centimetres are to small and kilometres are far to large
Meters or feet would be best, depending on where you live.
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.
Yard
20 inch
Millimetres.
Answer: meters
liters
meters
metres would be the best way to work out the length of a classroom as centimetres are to small and kilometres are far to large
the metric system
Centimetres.
A millimetre.
Meters or feet would be best, depending on where you live.
Either 'inch' or 'centimeter' would do the job nicely.