Each square is 720 meters on a side... or almost half a mile (.447 miles to be exact). Although not completely true, I go for the old "Two blocks equals a mile" measurement. It's not exact, but that's what I use when placing my mile markers. You can guesstimate the distance by eyeballing the track in MAP view and taking into account the curves and bends in your track. Of course, if your a complete perfectionist, this wouldn't work for you. Use the guides BOATDCTR is talking about.