To find the distance around the room you need to measure the lengths of all of its sides, then add them together. In your example if the room is 4m x 3m, the distance around the room would be 4+3+4+3 =14 meters.
But that would give you the result in meters, not square meters. Square meters is the measurement of the area of the room not the distance around it. To find the area of a rectangular room that measures 4m x 3m, you simply multiply the length by the width. So the are would be 4 x 3 = 12 square meters.
The height of the room comes into the measurement of the room's volume, which is in cubic meters. In the example above the volume of the room is 4 x 3 x 2.5 = 30 cubic meters.
Hope this help. Let me know if you have any questions.