The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), in Parashat Vaera (7; listen to audio recording for precise citation), discusses the intention that a person should have as he recites the verse of "Shema Yisrael Hashem Elokenu Hashem Ehad." He writes that in this verse we affirm two fundamental Jewish beliefs, which we introduce by declaring, "Shema Yisrael" literally, "Hear, O Israel." The verb "Shema," the Ben Ish Hai explains, actually means acceptance and understanding. (The Ben Ish Hai cites proof texts as evidence of this meaning of "Shema.") And in this verse, we call to the Jewish people to accept upon ourselves and fully understand the two tenets of faith presented in the rest of the verse, which are as follows:
1) "Hashem Elokenu" "Hashem is our God." These words affirm the belief in the fact that God rules the world, and we are therefore His servants and subjects. This is the fundamental concept of "Kabbalat Ol Malchut Shamayim" accepting upon oneself the yoke of Divine Kingship.
2) In addition, "Hashem Ehad" "Hashem is one." When we speak of God as "one," we mean "one" in the absolute sense, and not in the mathematical sense, according to which "one" implies the possibility of there being "two." Hashem is "one" in the sense that there cannot possibly be any being that even remotely resembles His abilities and stature.
Thus, when one recites this verse, he must have the following intention: "Understand and accept, O Israel: 1) Hashem is our God, and we are thus His servants; 2) Hashem is one, and there cannot be any other.
The Ben Ish Hai adds that one must recite this verse in a manner that reflects this meaning. Namely, one should pause after every two words in the verse "Shema Yisrael; Hashem Elokenu; Hashem Ehad."
The Ben Ish Hai further notes that the letter "Ayin" in the word "Shema" and the letter "Dalet" of the word "Ehad" are written in larger print than the other letters of this verse. He cites from the work "Keter Malchut" that when one reads this verse, he should visualize the letters, and he should visualize these two letters in a larger size than the others.
The reason why these two letters are enlarged is that they combine to spell the word "Ed" ("witness"). This reminds us that while reciting this verse we essentially testify to the fact that Hashem is our God and that He is one. Furthermore, the "Dalet" is enlarged to ensure that it does not resemble a "Resh." If one reads the word with a "Resh" instead of a "Dalet," he reads the word "Aher" ("other"), which might imply that God is "the other," one of two or more deities, Heaven forbid. To avoid such a grievous mistake, we enlarge the letter "Dalet" so that the reader clearly sees that it is a "Dalet" and not a "Resh."
Summary: When one recites the verse, "Shema Yisrael," he should have in mind the following meaning: "Accept and understand, O Israel, that Hashem is our God and we are thus His servants and that He is one and there is no other being that can possibly be like Him." Furthermore, it is proper to visualize the letters of the verse as one recites it, and to visualize the "Ayin" of "Shema" and the "Dalet" of "Ehad" as enlarged letters, as they are written in the Torah scroll.
