What about selective mail download?  Or IMAP.  The latter certainly works. 

If one sets a Download Control to limit the size of emails to be downloaded (using POP) then they are not downloaded -- this part works.  The problem is that the email is flagged as "read" by Pegasus, and even though left on the server, will not be downloaded in the future, even if the download size control is removed.  This is not good!

 You can get around this by temporarily unchecking the "download only unread email" box, but this is a bandaid solution, not a fix, and has the nasty side-effect of duplicating other messages already downloaded.

Pegasus should be fixed to NOT flag a skipped email as having been read.   

Found a workaround -- can use a POP server-side rule set based on message size, with a `leave on server` specification.  This `tricks` Pegasus into not assigning a message number and hence treats that message as still unread the next time it tries to download messages.  Removing the rule set allows the offending message to be downloaded normally.