I'll keep this simple
1/ We are
NOT going to implement a mathematical formula to determine vehicle size in the game. It is an enormous waste of time and man hours for more reason than I care to itemize but for one ANY "information" used to determine vehicle length would have to EXCLUDE gun overhang and a high percentage of specs give the maximum length that includes gun barrels and gun barrels don't figure in to a size calculation like that. What you will end up with is, in the vast majority of cases, the same numbers we already use as you have already discovered. I didn't need all the math to know that.
2/ I said "1-6 is the standard for tanks since SP1" as in little tankettes are one and a Koenigstiger is 6 and everything else fits in between with something extraordinary like the Maus being a 7 with most tanks fitting into the 4 -5 range.
3/ All of that diverts away from that "Most OOB's have many size discrepancies between vehicles and countries with same equipment" claim and I'm still waiting for the list.
Don