Those length figures come from the game's configuration files (.cfg).....found in each of the specific ship files of the "Data/Sea" folder.
The problem in using them is with the size of the view you'll be trying to measure these lengths with. The in-game views (periscope/TBT) are not accurately portrayed to give these dimensions an accurate measurement. It's like trying to measure with a yard stick that's not 36 inches long. What does it matter what the given lengths are if the tool to measure with is inaccurate?!
Read the "Discussion" section of the
Optical Targeting Correction mod for details (you'll find the explanations further down the first post, in the OTC thread). I'm always available to answer your questions.