Kilayim, Chapter 4, Mishnah 5. We have already learned that when a person plants a single vine standing on its own, one may not sow within six handbreadths of it. But when there are several vines, so that halachically they are considered a vineyard rather than a lone vine, the law is that one may not sow within four cubits of the vineyard.
What is the minimum size of a vineyard:
"HaNotei'a shurah shel chamesh gefanim" - one who plants a single row of five vines:
"Beis Shammai omrim kerem" - this row constitutes a vineyard, and therefore one may not sow within four cubits of it, and leaving six handbreadths is not enough.
"U'Veis Hillel omrim eino kerem, ad shyihu shnei shuros" - a single row is not considered a vineyard, and one may sow within four cubits of it as long as a distance of six handbreadths is maintained. Only two rows, each containing at least three vines, are considered a vineyard.
The scope of the prohibition:
One who actually sows within four cubits of a vineyard renders the vineyard forbidden, and the grapes become forbidden to derive any benefit from. The question is how much becomes forbidden, and this depends on the amount that is considered a vineyard, as the verse states: "Pen tikdash ha'melei'ah ha'zera asher tizra u'tevu'as ha'karem" - the seed sown there and the produce of the vineyard are what become forbidden. From the wording of the verse it is clear that it is the vineyard that becomes forbidden.
Accordingly, the dispute about the definition of a vineyard directly affects the scope of the prohibition: "Lefichach ha'zorei'a arba amos she'ba'kerem":
"Beis Shammai omrim kidesh shurah achas" - "kidesh" means that it became forbidden. According to Beis Shammai only one row becomes forbidden, since a single row is considered a vineyard, and it is the vineyard closest to the seeds that were sown alongside it in a forbidden mixture, and therefore it is the one that becomes forbidden.
And Beis Hillel - two rows become forbidden, since two rows are the minimum measure of a vineyard, and the vineyard planted alongside the seeds is what becomes forbidden. Therefore the larger area becomes forbidden - two rows and not just one row.
In summary: In this Mishnah, Beis Shammai and Beis Hillel disagree about the minimum size of a vineyard: according to Beis Shammai, a single row of five vines is already called a vineyard, while according to Beis Hillel two rows are required, each with at least three vines. From this dispute follows the scope of the prohibition when one sows within the four cubits of a vineyard: according to Beis Shammai one row becomes forbidden, and according to Beis Hillel two rows become forbidden.