Use this as a rough guide when someone asks you how large of a screen you need.
3-5 people 21 inches diagonal (53.34 cm) 5-9 people 29 inches diagonal (73.66 cm) 10-15 people 37 inches diagonal (93.98 cm) 16-35 people 60 inches diagonal (152.4 cm) 36-50 people 72 inches diagonal (182.88 cm) 51-140 people 120 inches diagonal (304.8 cm) 141-220 people 150 inches diagonal (381.0 cm) 221-390 people 200 inches diagonal (508.0 cm) 391+ people 300 inches diagonal (762.0 cm)
Source: www.bhphotovideo.com