It is capable of remaining in FOCUS while projecting a 300" image. The fact that the image is unwatchable is not their concern.
The general rule is that you want a properly treated 'blackout' theater environment for front projection and in that room you want about 14-17 lumens per square foot of screen space. As the lamp dims over time, up to 75% or more prior to lamp failure, the ability to recalibrate and maintain that lumen rating is very important.
Likewise, creating the right environement is extremely important and that includes providing a screen which is the appropriate size for the space you are in.
1080p video, when viewed from 1.5x the width of your screen, will be of near perfect clarity to average viewers with a good projector. Go much larger and people with average or better vision will be able to detect flaws in the image quality and could see individual pixels and screen door effect (SDE).
Go smaller, and you aren't getting the full benefit of the resolution that the projector provides to you.
In the end, it's a balance of personal preference and the conditions of the environment. In an ideal world everything is perfect - a very dark room, a 1.5x width viewing distance, and a projector with plenty of lumens to fill your screen properly throughout the life of the lamp.
The big plus is that if a projector does start to look lousy after 1,000 hours or so of lamp life, you can often replace the lamp and the projector will look as good as the day you got it. It's pretty incredible to see that happen.
