If projecting images from a computer screen, then today's LCD/DLP projectors will be rated either 800x600 pixels or 1024x768 pixels size. This means they project a computer screen of that size. What you do is to simply prepare a good image of that size, which views properly on your same-size pixel screen. An image larger than your screen is able to show will obviously not make it "better".
PowerPoint still works in inches, from the days of printing slides to view on overhead projectors. For screen projection today, create the photo images to be the size you want to see in pixels, for example create 800x600 pixels to totally fill a 800x600 pixel screen, or maybe 400x300 pixels to fill 1/4 of that screen. Provide sufficient pixels to show it on the screen at the size you want to see, so it won't be resampled very much, especially not to be larger.
If printing transparencies for the old style over-head projectors, the considerations are about the printer instead of a computer screen. You would print that transparency in the same way you would print any image on that printer. Just print the best image you can. You would scale the image in exactly the same way you would print any image on that printer.
In either case, you may feel it is important that the projected image may be ten feet wide (about three meters). That is true, but the original is either a computer screen, perhaps 1024x768 pixels, or perhaps it is a printed 8x10 inch transparency. You cannot do any more than to prepare the original well, the concern is about preparing the best original you can, instead of the wall.
However, the main rule for projection is to create images containing only big bold detail, because tiny details in that original will not show up in the rear of the viewing room. A ten foot wide image on the wall is 120 inches wide, and 1024 pixels spread over 120 inches is about 1024/120 = 8 ppi on the wall. So stand way back from the computer screen to evaluate the readability of an image to be projected. For example, viewing a one foot wide computer screen from 1/10 of the auditorium length simulates the viewing angle of a ten foot wall screen seen from the rear wall.