Look at the numbers on the sticker directly on the tube (not chassis). They will likely be axx then a bunch of other letters/numbers etc. Those two numbers directly after the "A" are screen size in cm. A68=68cm, which is approx 27".
Problem is a lot of Japanese company like sanwa don't use viewable space they will use total tube dimension. going off model numbers causes more confusion than good. Just go off the actual tube manufacture spec numbers.