Thanks a lot for this reply! "It's one of the most difficult issues in empirical economics." - this is what I suspected when I made this post. The link to the document you gave explained a lot!
I read this research 5 years ago (2005 version). The concept is great, though as they measure that number once in 5 years and so far we have 2-3 data points, it's quite difficult to use it in empirical research.
Now I see that you even didn't understand the question! There is capital stock term which is an accounting term and it relates to a company (this is what you assumed) and there is capital stock which relates to macroeconomics (which is what I asked about). Cobb-Douglas told you nothing. Why bother to comment and send useless links to Wikipedia? It turned out this forum has enough knowledgeable members.