1 pixel = 1 meter so 1000 meters is a km. so if you have a 8129x8129 a supported size you would have a 8.129 km map. km are .62 of a mile so youd have to either do a lil math or put it into a converter online but in miles comes out to 5.0511264. but why think in miles if your game engine thinks in terms of metric measurement. Metric is good because of the whole power of ten.