I wondered the same thing, but more related to the overall size of a competitor's site so I did some searching.
I found https://websitewordcalculator.com/
Seems accurate, but my only point of reference is my own website.
The calculator says I have 143k words and I actually have 140k.
Page count seems off though. It says I have 198, but I actually have 135. Feels like there is some wiggle room here with category and tag pages.
There are word counts for individual pages. Maybe that helps you?
I use the WP Word Count Plugin for word and post count currently to track my progress.
If I can find good stats online, I might get rid of the plugin.
I'll be following this thread because I would love to know better ways to size up my competitors content and track their growth.
@contract I wouldn't even bother with the average for the entire site.
You should focus on what keyword you want to rank for and then focus on page averages for that keyword. An entire domain's average is not that helpful to be honest. Even if you are buying that site - wouldn't you want to get more specific into the pages/keywords/content quality, etc..
I guess I am not sure why you need the domain average word count.
Another thing with averages is that they don't show you an accurate picture. Take this scenario for example: A site has 10 posts. 1 of them is 10000 words and the other 9 are 100. The average is going to look way higher than it actually is.