Just not comfortable buying a 'package deal' with a particular screen size and finding out it doesn't work well in my room.
I don't recommend throw it on the wall and see the size method. It doesn't account for that bulbs lose more than 50% of their brightness over their lifespan. So what looks great now, won't after you get some hours on the bulb. Bulbs tend to lose about 25% brightness in the first 500 hours and then gradually decrease over the remainder of their life. The size screen also depends on the type of screen and what gain the screen has. This should be chosen on your room characteristics and viewing habits. Are you going to watch with lights on? Color of walls and ceiling? Picture quality also depends on your viewing preference. You can start out in best picture quality mode and the change to brighter but not as good picture quality modes as the picture dims.
Also, I don't recommend sacrificing audio by going too big where it makes for poor speaker placement. You have some factors to consider in your choice of screen and size for your situation.