Our digital projector is a Canon SX80, with a maximum resolution of 1400 x 1050 pixels. 1400 divided by 1050 is 1.333 or 4/3, so the projected images have a format ratio of 4:3, which is also true of the 1600 x 1200 pixel size chosen by the PAGB as the maximum size for competitions.
The projector does not project images at 1600 x 1200, because it can't. It projects landscape format images at 1400 x 1050. 1400 times 1050 is 1,470,000 pixels, so a competition entry is apt to be about 1.5Megapixels, so generally under 2 Megabytes as a file. If your entry file is bigger than that, you might check it has been sized correctly.
So what dimensions does it use for a portrait format image?
Good question. The maximum possible height the SX80 can project is 1050 pixels. If the image height is more than 1050, it will be reduced proportionately to give a height of 1050.
Say the image is 1400 pixels high. 1050/1400 is 0.75, so the vertical side is reduced to 3/4 of the file size. To avoid distortion, the horizontal must be shrunk by the same factor. If the width happens to be 1050 it shrinks to 1050 x .75, giving 788 pixels. So a 1050 x 1400 file is shown at 788 x 1050, which is 827,400 pixels.
827,400 is only 56% of 1,470,000, so a 1050 x 1400 portrait, when projected, will be 56% of the size of the equivalent (1400 x 1050) landscape image. This should be remembered in competition as the portrait image may lose instant impressiveness as a result. What size a judge sees it as naturally depends what equipment he views it on.
There is no rule that the 3:4 format must be maintained in portrait images. As long as the height is 1050, the width might be set to (for instance) 900 pixels by cropping, giving a gain of 14% of area over the 788 x 1050 version. This is a matter of personal preference.