There is a little confusion from your post regarding Storage space for mail vs.
size of a mail message.

JUNO has a 60K max size message limit.  But I don't know how much space is
allocated for total mail.

The bounce you show from Yahoo is stating that the user is over quota (the
amount of total mail storage they are allowed.).

>The original message is over 5k.  Message truncated to 1K.
>--------------------- end of error message -----------------------

This portion of the bounce is Yahoo telling you that rather than return the
entire message in the bounce processing that they have truncated it and
returning to you a 1K of the bounce.

So the bottom line is the YAHOO mailbox of your subscriber is full and they
should be deleted or set to nomail.

Randy
Our-Kids