I think what he means is that having duplicated the layer its still main size.
what you need to do is resize the image to the size you want one picture to be
duplicate the layer 5 times
then image resize, canvas size - anchor top left (or bottom right or whatever anchor to one of the corners) resize the canvas to 3 times its current width and twice its current height
then use the move tool to move the various layers arround till you have 6 images laid out next to each other
then finally layer, flatten.
edit : 'Hypnotic' types faster than what i do