In a case of murder on a Greek island by a single defendent of a
single victim, non-terrorist related, would the accused be tried by a
jury of 12, or fewer, or by some other means?

Since this question has been around unanswered for several days, I
will venture to post a free comment that may help you.

The trial for the murder of Sotiris Yialamas by Akis Panou would be an example.

"[32] Songwriter Panou convicted of murder
Athens 24/03/1998 (ANA)
A Kavala court yesterday convicted a popular Greek songwriter on a
murder count, sentencing him to life in prison.

Akis Panou was also convicted of illegal arms possession.
Five members of the seven-member court also did not find any
extenuating circumstances for Panou, who admitted to killing his
daugther's married boyfriend last summer, claiming it was a crime of
"honour" and committed in the heat of the moment.
He has the right to apeal the verdict."
    http://18.85.3.140/cgi-bin/brief?/news/greek/ana/1998/98-03-24.ana.html#32

Note that the above refers to a seven-member court, either a "mixed"
Court of the First Instance or a "mixed" Court of Appeal, as described
in the following Wikipedia article, which explains that these are made
up of 4 jurors and 3 judges.  The higher levels of appeals courts
consist of 3 or 5 professional judges.

http://en.wikipedia.org/wiki/Judicial_system_of_Greece

I hope this free comment answers your question.

Thank you very much for your most helpful information.

Hello Tisviv,
My pleasure, and thank you for coming back to say so.
Cheers, Myoarin