Metres would be the best unit to measure the length of a backyard
A metre would be an appropriate unit.
A centimetre would be an appropriate measure.
the most appropriate measurement for an ant would be the millimeter
A tape measure that measure in feet and inches (or meters and centimeters) would be most appropriate.
The appropriate measure will depend on the object being measured. An appropriate measure for the length of an ant is a millimetre, but that would not be an appropriate measure for the distance to the nearest star outside the solar system! Likewise, a milligram is an appropriate measure for the mass of an ant but not so for the mass of an elephant.
A centimetre would be appropriate.