If your drive isn't connected, i Tunes will temporarily default to the original location until your drive is connected again.
To gauge the size of your media library, open i Tunes, select one of your libraries (music, movies, TV shows, podcasts, audiobooks, and so on) from the left pane, and take a look at the numbers written at the bottom-center of the i Tunes window.
You should see the number of files in that particular library, along with the size of the library, measured in gigabytes.
To walk you through it, here's a step-by-step guide on how to move your i Tunes library to an external hard drive.
This tutorial is also available as a step-by-step video.
Even though your old files are long gone, i Tunes will not delete its record of the file simply because it can't be found.