Projecting slides on a large screen from 40 feet I have a few small projectors for small rooms, but I'm finding that these do not work well for pushing light to a larger screen further from the point of projection. 

In looking for possible upgrades, what key features do I need to evaluate in comparative shopping?  Is there a metric or feature I need to look for in a projector that will allow it to throw a bright, clear video projection from further away?  To clarify, I'm not needing to replicate what the local cinema's use - I'm only showing slides not video content.
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)