20x20km is 400km2. This is really huge area. For example The Witcher3 has 218km2, GTA V 127km2. Your desired 30x30 can be compared to Just Cause 3 - 1036km2. You need to hire army of artist to fill that area with diverse and interesting content
Of course, you do not have to stick with the scale 1uu = 1cm. Some game genres dont need milimeter precision. But remember, further from 0 = smaller float precision
