Yep. 7' x 100' = 700'. Minus the air space between the strands is how you figure the filter area.
Think of it this way. You could have a rectangle that is 7'x 1,000' but it is only made of a thin strand of wire around the perimiter. Would you count all the empty space inside that rectangle as filter area?
That's why filter media is small strands woven together with air space between them, in 1/2" thick or so mats. More surface area per square inch for the microbes and bacteria to colonize.
Sand has a huge amount of surface area, but only if it's fluidized. If it's packed together tightly, just figure on the square foot area that the perimeter covers.
