When you've been growing your music collection in iTunes for several years, trying to figure out how much space it occupies and how much of it you can fit on your iPhone, iPod or iPad can be an exercise in frustration. By default, iTunes won't display size information for your music, but you can set it to show in one of two different ways.
The Status Bar
Click "View" and select "Status Bar" to show the status bar, which holds various information about your music. When you have nothing selected, it'll show how many items you have in your music library, how long it would take to play them all back to back, and how much space they take up in total. When you have something selected, whether it's an album, an individual song or a group, it'll show the same information about that.
The Size Column
Click "Songs" to switch the view to a list of all the music you have in your iTunes library. In this view, you can add and remove columns to display or hide various information about your music. Click "View," "Show View Options" and check the "Size" box to add a column that shows the size of each individual file in your library.