16 x 20 and 18 x 24 are two of the most standard poster sizes.
Centimetres, or millimetres if you want to be particularly precise.
Centimeters. Sometimes millimeters are used instead. Note that it is easy to convert from one to the other - just move the decimal point one place.
inches, feet, centimetres, metres
inches
meter
metric unit
miles?
Millimeters
The metric unit of length is the 'meter'.
mm
metric unit
kilograms
A microgram.
meters
Litres.
Tonnes
miles?
Millimeters
The metric unit of length is the 'meter'.
You would use meters.
I would use centimetres.
You would use a ruler and measure in millimeters or centimeters.