Same problem here. SomeSome POP3 serverservers are missing the LIST command. This command indeed gives the size of each mail available. To avoid this check, and soto find a path to restore the service, just use the fetchmail "fetchsizelimit" option and set it to 0.
fetchsizelimit 0
This way, fetchmail doesn't care about sizes any more, so skip and skips the use of the LIST command. Problem solved.