The first thing you need to decide is where to place the projector away from the screen area, AND how big is the screen area you have on your wall to use.

If you decide to place the projector around 2.5 metres away from the screen wall, then look for a projector with specification that have a Throw Range distance that covers that 2.5 metre distance. For example, the BenQ HT2050 projector has a Throw Range of 3.7m to to 4.8m so you cannot place this projector at 2.5 metres away from the screen because sharp focus image cannot be obtained... by moving the projector to 4 metres away from the screen wall then a sharp focused image can be obtained.

A Long-throw projector requires it to be placed quite far away from the screen. A Short-Throw one has range nearer to screen and an Ultra-short Throw one can be placed very near to the screen. When you use an Ultra-short Throw projector, do make sure that the projection light coverage is wide enough to spread to the entire width of the screen.

Because of the size of your room, I think you should look for Short-Throw and Ultra-short Throw projectors which stays within the distance that you want and make sure the light coverage is wide enough for your screen size.

If the maximum width for your screen is around 8ft, then the screen size should be around 80inch (diagonally) or 39inch high x 69inch wide for Home Theater 16:9 ratio.

https://www.engadget.com/2006/05/23/how-to-...n-home-theater/

In a completely darkened room, a projector with a lower Ansi Lumen should be enough to light the screen with proper brightness and black levels. Ansi lumen of 1500 should be bright enough to cover a screen size of 45inch  x 80 inch in an almost darkened room. When the room have lots of ambient light from the room lights or windows, then a much higher Lumen is needed. Usually the higher the Lumens, the more expensive the projector cost.

http://www.projectorpeople.com/resources/lumen-guide.asp

^Well explained by jamesleetech

Another site to check out the metrics for projectors is - http://www.projectorcentral.com/projection...culator-pro.cfm