Is there an easy way to list CLI tools installed on macOS?
Basically every directory listed in PATH besides the standard ones (/usr/bin:/bin:/usr/sbin:/sbin) contains user-installed, so running ls on these directories gives you the list for them and you can use the usual Unix tools like du to calculate sizes. 

To make managing user-installed tools easier you can rely on a package manager. Homebrew is the package manager of choice for many users.