The slides should not be designed for the computer screen, but for the projected screen. One difference is that unlike with the computer screen, the distance between one attendee and the screen is constant and cannot be changed.

I did the following experiment. I went in the back of the conference room and I measured the size of the screen as if it would be projected at an arm length.

The projected screen that I saw from the back was equivalent with a 20cmx15cm screen projecte at 60 cm.