Determining the full postgresql size on windows
Part of the difficulty here depends on setup.  For this purpose I will assume a default setup.  I am also assuming here that we aren't counting system libraries here, or connection libraries, only PostgreSQL itself (i.e. no dependencies, and not libpq).

The best place to start is indeed the sum of the user folder plus the program files/postgresql folder.  If you are putting the logs elsewhere you will want to add those in as well.