I would recommend about twice the height of the screen = the distance from the screen. so a screen of no more than 2 meters tall. So about 3 meter across (not diagonal) would be about the largest I would attempt.
Speaker placement then becomes a challenge, of course, so people with that large a screen often get an acoustically transparent screen and place the speaker behind it. You can reach some of the discussions about that.
What I am looking for is, where on the wall do I put the screen?
If I am sitting back 6.5 feet, with a my eye height at 39 inches, the, THX recommended, top of the screen should not be higher than 60 inches? (tan 15 * 78 inches = 20.9, then rounded to 21. 21 + 39 = 60")