My guess is that it is a 16".... measuring the neck glass and front diameter in the picture, I get about 10.6 for the ratio of neck to front diameter. Neck glass is about 1.45" on several CRTs that I measured, including a 16AP4. 10.6 x 1.45" = 15.37"... pretty darn close to 16" for crude measurements. By the same type of ratio measurement, the CRT in the picture appears to be at least 2 inches shorter than my 16AP4.
jr