Even in the latest (beta) version, mutt displays the size of all messages as "0" if they are in a Maildir.
I looked at the FAQ and it has a workaround that uses procmail to insert a "Lines:" header, but I'm wondering if there's a less kludgy way to do this. Wouldn't it be relatively easy to patch mutt so that it displays bytes instead of lines? In a Maildir, it only takes a simple stat() operation to find the size of a message. I'm more used to seeing bytes anyway since I come from using pine...