What are the Halachic boundaries of Eretz Yisroel?
There are two sets of boundaries that define "Israel" with respect to Halacha.
Generally speaking, the Western border is well enough defined as the Mediterranean. (Though see the Gemara in Gittin Chapter 1, and Tosfos' commentary ...)
The first set is the Biblical borders, as given in Numbers Chapter 34. The southern border is nahal mitzraim, generally translated as Wadi El Arish. Many printed translations of the Torah now include a map representing these borders according to the view of rabbinic cartographers such as Kaftor VaFerach.
The second, much smaller borders, are found in the Talmud, Mishnah Gittin 1:2:
From Rekem and eastward, including Rekem [is not Israel]; from Ashkelon and southward, including Ashkelon; from Ako and northward; Rabbi Yehudah considers Ako itself outside, Rabbi Meir disagrees.
The latter borders are the ones that define, for instance, "the land of Israel" with regards to agricultural laws such as tithes (terumot umaasrot) and the requirement for a sabbatical year (shemitah).
In his yutorah.org lecture on halachic issues related to the Gaza Disengagement, Rabbi Hershel Schachter points out that Gaza is within the Biblical borders (north of Wadi el Arish) but outside the Talmudic borders (south of Ashkelon). The halachic implications of that are a very lengthy discussion.
Well there are Biblical borders, and then there are the Talmudic borders. (Much of your question is addressed there). The Talmudic northern border is Ako (as given in Mishna Gittin Ch. 1). The Biblical northern border is addressed by Numbers 34:7--9:
34:7 This shall be your northern boundary. From the Mediterranean Sea, draw a line to Hor Mountain.
34:8 From Hor Mountain draw a line along the Chamath Highway, so that the extreme edge of the boundary is toward Tzedad.
34:9 The border shall then extend through Zifron, with its extreme end at Chatzar Eynan. This shall be your northern border.
Rabbi Aryeh Kaplan identifies "Chamath Highway" as the modern-day Homa, Syria. See his commentary for more. Most of the other locations identified by Rabbi Kaplan's translation can be found on Wikipedia and/or Google Maps.