50" is the diagonal.
Most screens on modern widescreen TVs are 16:9 aspect ratio. That means if it's 16" wide, it's 9" high. Using Pythagoras's Theorem, you can determine that its diagonal would be 18.358" (this is calculated as SQRT(16^2 + 9^2)).
Given that you're looking at a 50" TV, you scale everything by (50/18.358), or 2.724.
So the width of the screen would be 16x2.724, or 43.58", and the height of the screen would be 9x2.724, or 24.51". That's the smallest the TV could possibly be, but inevitably it has a frame around the screen, so Barry's 44.4" x 25.3" shows you how big the frame is in total (about 0.4" all the way around the screen).