The brightness you need is dependant on a number of factors. The main ones being: how much ambient light is there, how big is the screen and how far away will the people be who need to see the screen. The table below should give you a rough guide but it is always worth phoning up to chat through your unique situation.

low ambient light
some ambient light
high ambient light
	Screen diagonal (approx size)
Lumens	72" (5ft x 3.5ft) (1.5m x 1m)	100" (7ft x 5ft) (2m x 1.5m)	120" (8ft x 6ft) (2.5m x 1.8m)	150" (10ft x 8ft) (3m x 2.2m)
750
900
1100
1300
1500
1700
1900
2100
2300
2500
2700

P.S. Incase you were wondering about the definition of a lumen: lumen - a unit of luminous flux equal to the light emitted in a solid angle over a spherical surface by a uniform single point of light equal to one candle.