About
whistle size terminology, I posted this a while back on a related thread:
You'll hear different people using different terms for various whistle pitch ranges. It's inconsistent, and mostly unnecessary.
When I started playing all there were few whistles available:
Generations in (from lowest to highest) Bb, C, D, Eb, F, and G.
Clarkes in C.
And that was it.
These whistles were all called, simply, "whistles". There was no need for any other denominators.
Then, according to Finbar Furey
"Bernard and I, over the summer of 1970, designed and manufactured the first of the Overton flutes."
The title "flute" didn't last long, and soon enough people starting calling these "Low Whistles". Then the backformation "High D Whistle" was coined to avoid confusion.
Even today many/most people just use "high" and "low" because usually it's sufficient.
High: Bb, C, D, Eb, F, G (the old Generation keys)
Low: Bb, C, D, Eb, F, G (some of the new Overton "low whistle" keys)
"A" whistles could just be called that at first because there were no others. But then Overton started making huge Low A whistles, a fourth lower than the Low D.
And Overton started making huge Low G whistles, a fifth below the Low D, so now you had THREE octaves of G whistles.
(Here are G whistles in three octaves by Alba)

So what to call all these things?
Some continued to call the one in the middle the "Low G" and the huge one the "Bass G".
Some called the huge one the "Low G" and the one in the middle "Mezzo G" or "Alto G" or "Tenor G" or what have you.
I think some of the names are borrowed from Recorder naming practices, which is logical if you're a recorder player, but potentially mystifying if you aren't.