How to set my Nikon D90 to display a full image when I hit the playback button?

It is now set to only display a small thumbnail in the upper left portion of the screen and all the photo information in the rest of the screen.

RTFM

Press display whilst in playback mode

Arrow up or down with the multiselector to change the display mode.

Just press down arrow of the 4 way navigation button (and press it again until you get what you want).