Metres
Metres (in a built-up area), kilometres if across open spaces.
meeters?
The answer depends on what characteristic you wish to measure: its mass, length, volume etc.
It depends on what you mean by size: its length, volume, surface area or even its mass.
Miles is best.
Metres
Meters should be used to measure the length of a tennis court in metric units.
I would use centimetres.
You would use a ruler and measure in millimeters or centimeters.
kilometers
feet
Centimeters
mmeters
Centimetres
centimeter
Millimetre.