[Arran]

The image on an early black and white TV isn't blurry at all if it's working properly, at least not on a TV with a magnetically deflected picture tube. On the electrostatic sets the picture can get a bit off once you go above about a 7'' diameter screen, but it also depends on the circuitry used in the set.
I find that a 10 inch TV is quite watchable, you have to remember that these have round picture tubes not rectangular so the measure is closer to a horizontal width rather then diagonal like a rectangular tube. The reason most were 10 to 12 inches is because of the deflection angles, when you get larger then 12 inches you end up with a very large and heavy bell to deal with, and a longer envelope to boot. This was part of the logic behind the metal-glass picture tubes was to cut down on the weight. Some sets used rear projection to get around this problem, they used a small CRT and lens to blow the picture.
Regards
Arran