:: scuffs toe of shoe; smiles shyly ::
Wouldn’t the longest thread be the one with the largest word count?
What about one with fewer words than that one would have but using bigger words and so more letters?
When is the standard number of letters per word when wordcounts for books are announced?
The phrase you’re looking for is “column inches.”
There’s no one good answer for this. I’ve seen five letters per word, or six characters (the space after is the sixth.) In the days of typewriters, standard was 250 words per page with pica type (10 characters per inch) or 333 words with elite type (12 characters per inch), both approximations. That was when ms style was fixed at 25 lines per page with one inch margins. You can’t do that with a book, because every book seemed to use slightly differently-sized type, with different margins, and different leading. That was often manipulated to keep books from getting too short or too long - the physical thickness of a book and its word count are only mildly correlated.
You have the same problem online, since people use different default fonts. Length of an online thread is also affected by the number and size of quotes. Might as well use number of posts as first approximation. But not number of pages. That’s right out.
Someday, I’ll have to check what this thread is about. I remember that it started with actual pigeon eggs, but I assume that after several years, the chicks were born and have flown away long ago.
No, the one with the largest word size!
We keep getting more. Right now we have a pair of chicks, or “squabs,” out there that are 12 days old. 
Oops! They’re only 11 days old. I thought it was Wednesday. (It’s Tuesday morning over here right now.)
Which forum does that properly belong to again?