Frontal area is the size of the hole you have to punch in the air to drag the trailer through the air. Basically on a TT, it's the measurement of the front of the TT in sq ft.
If you exceed the frontal area limits, that just means that your aerodynamic drag will be high enough that you cannot tow at the GCWR at normal highway speeds without overheating your drivetrain. Yeah, as SkiSmuggs
suggested, just ignore that limit, but don't be surprised if your truck is reluctant to drag that trailer at 75 MPH without fussing at you.
My 7x14 cargo trailer box has frontal area of 56 sq ft, plus the wheelwells stick out on the sides, and it drags a bunch of air. I usually tow at 62 MPH, and my F-150 tells me that's all it neees to be pulling through the air.