You might be able to determine a rough size without having to download all the images. If you download the HTML page, you could then get a HEAD for each image it has, and add up all the reported sizes. Of course, this relies on the web server reporting a proper size, and I don't know how reliable this is, but it seems like it should be pretty good. (Doesn't HTTP use the Content-Length to determine how much to download? If so, then the Content-Length would probably be pretty accurate.)

HTH

Update: after re-reading, I'm not sure if I really answered your questions at all, but this is a thought I had. Perhaps it will at least give you some ideas...