Question: May one sit with one’s legs crossed in the synagogue?
Answer: Rabbeinu Yitzchak of Corbeil writes in his Sefer Mitzvot Katan (end of Mitzvah 11): “Woe unto those who engage in idle chat or act frivolously while in the synagogue during prayers and by doing so, prevent themselves from entering the World to Come. If gentiles who do not believe stand in utter silence in their houses of prayer, this certainly applies to us when we stand before the King of All Kings.”
This means that the behavior of the other nations of the world is to respect their houses of worship and they sit there respectfully. If so, we must certainly act in this way when sitting in a synagogue which is considered a miniature Bet Hamikdash and we must act respectfully and politely when inside.
In his glosses to the Sefer Mitzvot Katan, Rabbeinu Peretz writes: “When sitting and praying, do not outstretch your legs or put them one on top of the other (cross them), for these are considered arrogant behaviors.” Based on the words of Rabbeinu Peretz, one may not sit cross-legged during prayer services. The Mishnah Berura (Chapter 95, Subsection 2) quotes this opinion.
Similarly, Hagaon Rabbeinu Yosef Haim zt”l writes in his Sefer Ben Ish Hai that one should not cross one’s legs during prayer services. The reason for this is sitting cross-legged is impolite and is a sign of arrogance. Furthermore, the great Rishon Le’Zion, Hagaon Harav Yitzchak Yosef Shlit”a writes in his Responsa of the Rishon Le’Zion (page 51) that it seems that it is forbidden to sit cross-legged any time one is in a synagogue, even if it is not during prayer services.
Indeed, when Maran Rabbeinu Ovadia Yosef zt”l would see people sitting cross-legged in his synagogue, he would point out to them gently that it is inappropriate to sit in this way in the house of Hashem.
Summary: One should not sit while crossing one’s legs in the synagogue during prayer services. Furthermore, it is inappropriate to sit in this manner in the synagogue even not during prayer services if there is no real necessity to do so.