I’ll try and answer that as ‘basically’ as I can, LOL. The good-length spot varies from batsman to batsman, depending upon his height and reach. The good-length spot for a shorter batsman will be closer to the stumps than that for a taller batsman. Generally, though, the good-length area can be indicated as between six feet and twelve feet from the batsman for a fast bowler, and between four and six feet from the batsman for a slow bowler.
Yeah, a batsman plays fuller deliveries off the front foot and short deliveries off the back foot. A good-length delivery lands on a spot not as close as the one on which a fuller delivery lands, nor as farther down the wicket as the one on which a short ball lands. Therefore, it challenges every batsman.