The Options dialog box has selection of predefined size for drawing area, and based on the selected drawing area size Freestyle automatically calculates the scale value. Unfortunately, currently you cannot customize drawing size nor choose the scale value.
Please note that you can log your feature requests on Freestyle wishlist, where you can also see and vote for other Freestyle user’s ideas.
One other trick that gives you a few more options is to play with the drawing size once you have started.
Go to the file menu and select Options > Drawing Size. You can then change the paper size. While this will not let you select scale and drawing area independently, it will give you a few more configurations from which to choose.