A succah that does not have a minimum of seven by seven tefachim is prohibited. There is, however, no maximum size (Shulchan Aruch, section 1). See above, chapter 9, question 1, for an explanation of the term tefach in modern usage.
2. How do we know that the minimum size of a kosher succah is seven by seven tefachim?
Although laws which apply to "houses" in reference to the laws of mezuzah, maakeah, leprosy, etc., do not pertain to a structure which measures less than four by four amos. We have a tradition, however, that for a succah, which is considered a temporary dwelling, seven by seven tefachim would be enough. This size is sufficient to contain one's head, most of his body, along with a small table consisting of one square tefach. With this he can fulfill his obligation of succah (Aruch Hashulchan, section 1).3. What would the halachah be in reference to a succah whose width is less than seven tefachim, but whose length is greater than seven tefachim?
Neither the length or width of the succah can be less than seven tefachim.4. What would the halachah be in reference to a large succah which has one corner that is less than seven by seven tefachim (see diagram)?
In this case there is a disagreement among the halachic authorities. The Mishnah Berurah (section 1) quotes the Magen Avraham that even in a large succah, if in one corner there is an area of less than seven by seven tefachim it is prohibited to sit in that area, because this area is too tight for sitting. This same opinion is brought down in the name of the Elia Rabah.Next Chapter
The Chazon Ish brings down (chapter 144, section 6), that it would have the same halachah as the principle of písal hayotzei min hassucah, which teaches that an extension of the succah is considered as the succah itself (see above, chapter 7, question 12; chapter 9, question 13), for in regard to any area that is subordinate to a large succah we are not concerned if that area itself is not large enough for living activities, for as long as it is subordinate to a proper succah it is also considered as part of the succah.