>>56852810spreadsheet made by the developers in the leak that literally lists all the official pokemon sizes (up to gen 7) and what they actually mean.
There's a couple birds measured by LENGTH, and many that include their crest in the measurement (lol), but not a single one is measured by wingspan for its official size.
I went through and compiled all the pokebirds in it, here's a machine translated version (this took so long)
https://pastebin.com/Y9wQ9RDcand here's the whole shebang, which is a bit more broken but it has it all
https://pastebin.com/dqrti2cS 