I don't believe that the content-length of each individual file is sent by the browsers, only a total content-length for the entire upload. That means you won't know the size of each file until it is uploaded.

Your easiest solution is to just show the percentage of the entire upload that is finished, and then just update the name of the file that is currently being updated (which I believe the upload hook will pass to you). It might be nice to list all the files that have completed with the total size of each file as well.

Last year I did a little playing around with providing a progress bar for file uploads that may be of interest to you. Not sure if it still works, since I haven't looked at it in a while, but all the source is there for you to hack on.