Two sizes were supported according to the Wikipedia. My guess is one was 25" and the other was probably 19" or something close.
There wasn't really a standard. But most folks had something close enough to one of those two sizes and you could adjust the picture to fit if you had to. Depending on your TV. Black and White televisions tend to have controls that let you grow and shrink the picture in the tube. I don't know why exactly but probably as some form of compensation because the signal would be so erratic back then.