What is "u'Fe'as Yam"?
Rashi: The western border. Radak ? it is ha'Yam ha'Gadol.
Where is "ha'Yam ha'Gadol mi'Gevul Ad Nochach Levo Chamas"?
Rashi: From the south[east] corner mentioned above, i.e. Nachal Mitzrayim, until the northwest corner ? Hor ha'Har. It is opposite Levo Chamas, which is also in the northwest corner ? "Min ha'Yam ha'Gadol Tesa'u Lachem Hor ha'Har; me'Hor ha'Har Tesa'u Levo Chamas" (Bamidbar 34:7-8).
Radak: This is the width of the [western] border.
Malbim: The western border begins in the south, where the southern border ends.
Why does the verse conclude "Zos Pe'as Yam"?
Radak: This side mentioned is the western side of Eretz Yisrael.
Malbim: In the other directions, it said 'v'Es Pe'as Ploni', for the border can expand 1 . Here it does not say so, for the border cannot extend past the sea.
Radak (48:1, citing Yalkut Shimoni Ha'azinu 944): Hashem will make us inherit from one end of the world to the other ? "Hashem Badad Yanchenu" (Devarim 32:12).