I will try to explain hope it helps.
First off years ago the television sales guys started to advertise screen size based on the physical dimension from the outside of the glass corner to opposite corner.
This way it made it sound like a bigger screen.
Along came the consumer protection groups and said it was a no-no. Just because you have this much glass, does not mean that is what people see. They dictated it must be measured as the distance corner to corner of the actual lit portion of the picture.
This is where the term 25 V meaning 25 viewable came about.
Likewise this is why you here 29Inch and 27Inch th 27 inch should really have been 27V.
Now to complicate thing further, along came the tube engineers- in the case of sizes like 33" outside glass that became 31V. They figured out they could add more phospor where the outside black edges of the tube were. Now the lit portion could be a little bit bigger and become a 32V.
All this said, if you wnat to know the size of an existing tube- look at the tube label (usualy on the left bell of the tube - as vied from the back).
Usualy a letter then 2 numbers such as A68.
The 68 is the tube size in centimeters, just convert it to inches.
Hope this helps
Rick Nieman
Rick@Niemandisplays.com
www.niemandisplays.com