It will adjust to the size of a grid unit, whether you change the number of feet or set it to miles. If you want to keep the token scaled to the map, you could set your scale to .5 (35px per square) and set the distance per grid square to 5 ft on the larger scale map. That would keep you at 5 ft. grid units for measurement, keep the tokens scaled to the map features around them, and keep you from having to resize the map and dynamic lighting (if you are using DL on that map). If you aren't using compact token bars, then scaling down your grid that way can make the bar numbers not display very well.