What does "Midah" refer to?
Rashi: Its Medod (measurement). This is masculine, so it says Midah, and not Midasah.
What is the significance of "Aruchah me'Eretz Midah"?
Orchos Tzadikim (Sha'ar ha'Teshuvah DH Avur): The Avodah of Torah, fear and the attributes that man must acquire are very great.
Malbim: In understanding Elokus, both (abstraction and Hagbalah) are included. He is the ultimate abstraction, but also includes every perfection. Man is limited; he cannot fathom the distance between these two - "Aruchah me'Eretz Midah."
Daf Al ha'Daf (Shabbos 118b citing the Belzer Rebbi): No one finishes learning.
Why did he add "u'Rchavah Mini Yam"?
Devar Eliyahu: There are three depths that are four - height, depth, length and width. These correspond to the four letters of Hashem's name. Corresponding to Keser Malchus he said "ha'Cheker Eloka", "Im Ad Tachlis Shakai..." (7) - He has no end or limit. Aruchah and Rechavah correspond to written Torah and oral Torah. Height and depth correspond to Chochmah and Binah of Torah. Corresponding to Ma'ase Bereishis and Ma'ase Merkavah he said "Mah Tif'al" and "Mah Teda" (8). Via Binah man acts in worlds [even above] - "vi'Ydei Adam mi'Tachas Kanfeihem." Man's hand does not rule over Chochmah. The crown of Chochmah is revealed in the Torah.
