84 is a pure number and, as a number, it has no dimensions.
72 x 84
72" x 84"
The dimensions of a queen size water bed mattress is 60 by 84 inches. A king size water bed is 72 by 84 inches.
Multiply the two dimensions to get the area. In this case the calculation gives you 84 square feet.
You can't tell the dimensions from the perimeter. There are an infinite number of rectangles, with different dimensions, that all have the same perimeter. If it's 168, then the only thing you can be sure of is that the length and width add up to 84, but you can't tell what either of those dimensions must be.
72" Wide x 84" Long
One dimension is 84 inches. For the others, you would need to measure the sign.
None. You need a third dimension to get a volume.
That depends on how many faces this particular prism has.