70 pixels = 1 unit = However many feet, yards, or miles you want. Ideal situation you'll have more than 70x70 pixels worth of data in your map per square, so rather than stretching the map, you can use it slightly condensed. This allows it to still look sharp and great at 150% zoom to 200%, which is sometimes used in-game. If you have somewhat more or somewhat less resolution, it can still work fine (by stretching) and looks fine (at around 100% view). The settings for this are on the Page settings, so when you are GM, go into the Pages bar at the top (blue drop-down button), then click the little gear next to the page/map you want to modify its scale.
