i used to have braids down to my mid back and then the ends were knotted and soaked in hot water to seal in order to reach my waist. they used kanekalon hair (probably 48" in length?) for my braids, and since the hair isn't as long as my desired length, they would take a piece of hair for the first braid, and sort of stagger the strands for them to appear longer. so the ends on both sides of the length were uneven and thinned out at each end but looked longer. (does that make sense?) then, my braider would fold the hair in half around my actual hair and start the braid.
i'm sure someone who's really good at braiding will chime in but i wanted to share my experience since i did have super long braids.