Contrast
Contrast is a ratio between the lightest and darkest sections of an image. It comes in a ratio format, such as 400:1 representing the number of steps the projector can take to go from black to white along the greyscale. Generally, the higher the ratio the blacker the blacks will be. While contrast is always a factor to consider, it is more important with video and animated presentations. Data and basic graphical information can be displayed adequately on a 400:1 or 500:1 projector, whereas you may want a ratio of 1000:1 or more if viewing movies or presentations with video and animation.
Image size
The size of the image is also something to take into account. Depending on the size of your screen, and the number of viewers you will want to cater your image appropriately. Most projectors will give a range for the image they can produce, something like 40in to 150in diagonal and if you go above the maximum projection size you will start to experience image quality problems mostly resulting in loss of definition and focus.
Throw Distance
Part and parcel with image size is throw distance. The minimum throw distance is the closest the projector should be placed to the screen, and will result in the minimum image size. The maximum throw distance results in the largest image possible without loss of detail. Throw distance can also be referred to as either a short or long throw. A projector with a long throw needs to be placed further away from the screen than a projector with a short throw distance in order to achieve the same image size. Due to their primary use in boardroom presentation, most business projectors will have a short throw distance capable of producing a large image while being much closer to the screen.
Portability
Many projector buyers like to use their projectors in multiple locations, making size and weight a major factor. For those on the go, projectors that are easy to transfer and that can connect simply and quickly to a notebook is of utmost importance. Typically a portable projector is something that weighs less than 3 kilograms. This is light enough to be carried with ease around a large office, and setup in multiple locations. If you want something even smaller that can be carried on a plane or in a notebook bag, micro portable projectors typically weigh less than 1.5kg. DLP projectors are often smaller than their LCD counterparts, because they use a single chip instead of three LCDs.