Why does it matter?
If you know the dimensions of the screen, you can change the setup in PowerPoint to make best use of screen real estate. It also makes the overall presentation look different (and better).
I presented at a press meet last week where the screen size was 6 feet by 4 feet. I figured this out before I started making my slides. 6 by 4 also happens to be the most common screen size here in India.
PowerPoint allows you three setup choices; 4:3, 16:9 and 16:10. 4:3 is the default choice.
Screen size of 6 by 4 means a ratio of 1.50 (6 divided by 4). 4 by 3 essentially means 1.33 (4 divided by 3). The size closest to 1.50 is 16:10. Hence, if you choose a 16:10 setup the presentation will come out the best. It will look a bit different at first, but trust me it will look very refreshing for your audience. Try out a 16:10 and see the difference.