I'd like to see how big my library/playlists/albums/songs are in terms of file size. I plan on getting a WP7 phone and will only have 40gbs of memory, and I can only see how big my library is in terms of hours and minutes, not file size.
You can do this in Windows.
Assuming your music collection is in one folder (by default the Music library folder), you just right-click on the folder and select 'Properties'. The properties window will show you how much data is in that folder and all subfolders combined. If your music is not all contained in the same folder you would have to add up each separate folder individually.
Also keep in mind that if your music is high bitrate (i.e. large file size) you can set Zune to compress the music to smaller file sizes to fit more onto your phone (with some quality loss, but you probably wouldn't notice it).
40GB is a lot of space if you're just talking about audio (video is another matter). You should be able to fit something like 5,000 songs into 40GB of space. I have 2829 songs and 35 videos on my 32GB Zune HD and over 1GB of free space.