Here's a basic spec here:
http://www.projectors.co.uk/pp/Porta...I_CPR_S55.html
It's designed more for presentation as it has high lumens and low contrast.
Depending on how many hours it has on the lamp and whether or not it is set for video (more accurate colours) will have an impact on how bright it will be. If you want a cinema like image brightness, then you can go quite large on screen size but it will depend on how far back the pj can physicaly go in your room. A smaller image will be brighter. In the above link there is a guide to screen size and distance for pj to screen which should help. An easy method is to just fore it onto a wall and see what size is most suitable for your preferences, then get a screen to match it (if you really need one that is, a white wall can work quite well sometimes).
Gary