13.5'' x 13.5''.  Celebrated on 6th day until the end of the 7th of Sivian or Sunset on 14th until Sunset on 16th of May in 2013 by the Gregorian calendar.

Shavuot (or "Festival of Weeks" in English) is one of the Three Pilgrim Festivals and signifies the end of the seven-week "Counting of Omer" that begins after Passover.
Shavuot celebrates the gift of the Torah to the Jewish people at the foot of Mount Sinai.