First, I think it's great that you're buying a projector. You'll have the ability to have anywhere from a 30" screen to a 300" screen. Not only that, but averaging less than 10 pounds means its portable, too. But, before you make a purchase, it is going to be a good idea to do a little bit of homework.
First, you'll want to learn what a Throw Ratio is. This is the ratio of the width of the image being created and the distance to the projector. For example, let's use the W1070. It has a throw ratio of 1.15-1.5. This means that at a distance of 10 (120") feet you will get a screen width of 80" (92" diagonal) to 105" (120" diagonal).
A good resource for this can be found here:
Another important distinction is our Short Throw projectors. Our Short Throw models will always end in ST and are made to be placed closer to the screen. Since these frequently don't have a zoom, it will be important to use the calculator to correctly determine the distance from the screen before you begin mounting.
Next, you'll want to learn about offset.
In this diagram, offset is represented by <E>. Now then, the offset for this projector is 110-130%. If the start of the image was in line with the lens of the projector, it would have an offset of 100%. If the lens was lined up with the middle of the image, it would have an offset of 0%.