Constrain by surface area in Fusion 360
After some testing, Fusion does have a category to put in an Area parameter [edit -&gt; Parameters -&gt;] for Area that takes measurements in acres and circular mil but no useful measurements such as square centimeters or square inch. One circular mil is defined as a circle with a radius of 1/1000 inch, or 0.0254 mm, for an area of $5.067×10^{−4}\text{ mm}^2$.
However, defining the area of an item isn't (currently) straightforwardly possible: areas are not defineable, and with the obvious lack of possible parameters (square metrics and anything but super tiny or super huge) it doesn't seem to be planned. The best you can do is for bodies that you know the formula for the area in the following fashion:

Here, the left measurement (d2) is 1 mm. The parameter area is 10000 circular mil. Since we know A=d1*d2, we can go A/d2=d1 for a rectangle.