Not every fuzzy proposition is necessarily a social construct. Your example of a tall object is as good as your definition of "tall" is, but once that is defined any object can be measured objectively.

A good way of determining if something is a social construct is to ask yourself if the proposition would still hold if one were to remove all social lifeforms from the universe. A "tall" object would still be tall (to the degree that we can define tall) and it could still be measured somehow. In my example talking about a currency or an exchange rate wouldn't even make sense if we don't have a society that exchanges money or goods.