What is the area in square millimeters of a rectangle

Area of a rectangle is arrived at by multiplying its length by its width.

A = l * w

Make sure that both length and width are of the same units. 
for example
length is in feet, then width should be measured by feet

If both sides have a different unit, where length is in cm while width is in mm.
You can either convert cm to mm or mm to cm.
Just get the equivalent of 1cm to mm or vice versa and multiply the equivalent to get the desired measure in either unit.