Navfree required space
As you had said yourself, the maps size can be known only when you start to download.  The app itself takes approx 20Mb (Total size = App size + data) and it includes no map (actually app will say installing World map, but unless you download a country's map you wouldn't be see anything on screen that resembles a map).

Map for France:


You might also be interested to download other stuffs other than map. For instance for UK, you can download postcode database which can be very useful for locating a destination without Internet connection. Also, it would be worth while to download speed cameras database so that you would get a warning when you approach one and warn if your current speed is high to cause you trouble.