If you are able to use fractions - or smaller units - there is never a need to do so.
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A metres, for an approximate measure, a centimetre for a more accurate measure.
The meter.
A kilometre. A megametre may be more appropriate but it is not a unit that is used much.
The answer depends on what you wish to measure: its length, thickness, mass, elasticity. There are many more characteristics that you may wish to measure and in most cases a different unit is required.
A liter is a measure of volume, a millimeter a measure of length. There is not comparison