Using the formula of an area and then plugging in the width, and the length.
Assuming this room is in a rectangular shape
1) Measure the length of a room. Suppose it is 6 ft
2) Measure the width of that room. Suppose it is 7 ft
Applying the formula of an Area of a Rectangle:
S = w * l
S = 6 ft x 7 ft
S= 42 ft²
So in this example, the area of this room is 42 ft².