Morkus from Orkus
Lightning weaves back and forth, so while the line covers a 5 foot wide area, the bolt itself is smaller and zigzags within that area. A dex save would indicate getting out of the way within that space. If it didn't do that, there could be no save. It would be impossible to avoid. Successful dex saves don't make you fall prone, so you don't have to get under that 2.5 foot space to be successful.The lightning bolt is 5' wide. If we assume it passes through the center of the space, then to duck under it, you need to be less than 2' 6" tall. A 6' man doesn't duck under it, they would have to fall prone.
A Halfling, on the other hand, could duck rather easily.