The DVD player has a HDMI output?
Unless DVD player is upconverting or is Bluray your max resolution 720p which is a
a picture half the size of your HDTV, hence the small square. Use the TV controls and strech the picture for full screen. ( you have to like short fat people)
Otherwise you need to move the DVD plater or purchase a longer cable.
Also note the aspect ration of the source DVD. If it's 4:3, then the "square" image is normal. If it's 16:9, 16:10, or 2.35:1, then your DVD player is probably not upscaling to match your HDTV's screen resolution.