Projecting slides on a large screen from 40 feet
You'd probably want to look for the following characteristics:


Min and max projection distance (from the data sheet, make sure the projector can focus correctly for the desired distance)
Lumen (some people recommend 500 lumen per square meter projection area, but of course, this also depends on your ambient lighting)
Native image resolution (projecting on a large area the image may become blurry with low resolution)
Diagonal image size (make sure the resulting image size fits your intended projection area)