Go to LinkedIn and see how many employees work for each company. You can often see a breakdown by type of role on the company page as well.
Thanks. For Trello as an example, I can see 95 employees on Linkedin. I couldn't find any breakdown by role, but anyesy that number is somewhat helpful. Max 95 developers then
Investors track this sort of thing, your best shot is there.
Good pointer, I got a friend working at a vc maybe he knows something.