Reading the Megilla with many people



I pray in a synagogue close to me with about twentysomething members.  There is another synagogue close to my house which is much larger, about one and hundred fifty members, but I prefer to pray in the smaller one. Someone told me that on Purim it is important to pray in a big minyan, because the more people there are the greater the Mitzvah is.   Do I need to pray in the larger synagogue where Ima not a regular, or can I just pray in my regular small minyan?


Yes, you are required to hear the megillah being read in a large Minyan rather then a small minyan becase there is a greater mitzvah in large numbers of people.
The Gemara in Megillah (5a) writes, “Rav stated: If one reads the Megillah on the correct day (14th or 15th) one may read it by himself, but if it was read earlier then one needs ten. Rav Asi argued that no matter when one needs ten to read it.”
The BeHa” g ruled like Rav Asi that one must always seek out ten men to read the Megillah.
Rav Amram Gaon ruled that it is preferable to read the Megillah with ten men, but if one can not find ten men, he may read it by himself.
Rabbeinu Tam ruled like Rav that one does not need ten men when reading the Megillah on the correct day, he may read by himself if he prefers.
The Ro”sh and subsequently the Shulchan Aruch (OH 690:18) ruled like Rav Amram Gaon.
We see from here that one needs at most ten men, a minyan and not more. The Magen Avraham however writes that that ten men suffices when there is is no Bet Knesset or larger Minyan around but if there is then he needs to go there even if he has large minyan in his house.
The Mishna Berurah (690:62) brings the Magen Avraham and adds hat therefor even if one lives next to Bet Knesset and can here the Megillah from his house he is still required to go to the Bet Knesset because of “B’Rov Am Hadrat Melech-with more people performing a mitzvah there is more glory for the King.
The Biur Halacha quotes the Levush who writes just the opposite that as long as there is ten men in the Bet Knesset and there is Pirsumaei Nisa- publicizing the Purim miracle- and one may stand outside and listen. The Biur Halacha points out that for Pirsumei Nisa ten men is sufficient but he also has a mitzvah of Rov Am, performing a mitzvah with multitudes., therefore he must enter the Bet Knesset and listen with all the others.
To emphasize there are two obligations: 1) Pirsumei Nisa 2) B’rov Am Hadrat Melech.
There is another Gemara (Megillah 3b) which tells the story of that the students of Rebi left in the middle of learning to hear the Megillah in a Bet Knesset. That is even though they could have read it in the Bet Midrash where there were many people, they went to the Bet Knesset. The Ran explains to fulfill Brov Am.
The Mishna Berurah (687) writes that they were in a house not in a Bet Midrash so they had to go to the multitudes, but if there were many people in the Bet Midrash, they can read the Megillah there.
R’ Ovadia Yosef ruled: the best way of reading the Megillah is Brov Am, even to stop learning. That was why the students went to the large Bet Knesset, because of Brov Am. So, should we.
The two exceptions are: One is permitted to forgo B’rov if a sick person needs a minyan in his house.
One can pray in a small Netz Hachamah (sunrise) minyan.