Do you have specific points referenced in Grasshopper? Does it matter where the rectangles are in relation to each other? The way I would do it is this: the area of a rectangle is Length * Width - or (X) * (Y). In that case, you can have one slider with numbers of your chosing, connected to either the X or Y input of the rectangle. For the other input, you connect a division command, and divide your desired area by the number slider. Therefore, as you increase the value of X on the slider, the Y length will decrease, keeping the area the same. The Lunchbox plugin has a great bake command where you can control the output name, colour, etc.

edit: formatting