How do you calculate the dimension of a rectangle?

The dimensions of a rectangle, usually, refer to the length of the external sides. Calculation is not necessary in most cases, measurement is. Calculation is necessary when for instance you are given a set of dimensions and asked a question related to that information examples are;

What is the area of a rectangle that is 4 x 4 units

What is the length of the other side of a rectangle if one side is 2 and the area is 12