Well, in my quest for this answer, I found another problem - not all my album artwork is embedded in the track.
I happened to stumble over this when I was using Jaikoz Audio Tagger to clean up some of my existing tags. When I'd load an album's tracks into Jaikoz, it wouldn't show they had any album art. I thought this was strange, since iTunes showed it, Get-Info showed it, and Finder showed it. So at first I assumed that it was a Jaikoz problem, but the developer looked at the sample track I provided to him, and said there really was no embedded artwork!
The dev suggested that iTunes had the artwork in the ~/Music/iTunes/Album Artwork folder, and was using the artwork in there for iTunes' and Finder's thumbnail displays. After investigating a bit, it turned out that he was right. When I first brought my ALACs (from FLACs) into iTunes, I had iTunes download the album art while it was doing the import. What I didn't realize was that the album artwork wasn't embedded in each track, it was stored separately. I'd always assumed that iTunes embedded it, and when I subsequently added new tracks, I always embedded the album artwork using iTunes, XLD, Max, etc.
After realizing I had made a grave assumption, I bought PowerTunes to correct the situation. It has a function to take the downloaded album artwork and embed it in the tracks. Currently, it looks like 1/4 to 1/3 of my total tracks (24K) do not have embedded artwork. This is a staggering number since I had always assumed it was zero 🙂 PowerTunes also creates a nice log of the changes it finds, so I have documentation if I ever need to go back and fix one of the tracks.
So, my initial question still stands - how to determine the size of the embedded artwork, but in investigating this question, I found a big problem with my library. C'est la vie.