Question:

Why are the final letters of the first and last words of the Shema larger than all the others in the Torah scroll?

Answer:

One of the many explanations is that we enlarge those letters to ensure that they are read properly.

If the word shema, שמע, would be read with an aleph—which sounds very similar to the ayin—the meaning of the word would change from “hear” to “maybe,” changing a firm declaration of belief into an expression of doubt.

Similarly, if the ד (dalet) of the word echad, אחד, would be mistaken for a ר (reish)—as the two look almost identical—then echad (“one”) would be read acher (“other”). This would make our belief in one G‑d look like a belief in two gods.

Another explanation the commentaries cite is these two letters together spell עד (eid), the Hebrew word for “witness.” When we recite the Shema, we attest to His primacy. This reflects the words of Isaiah (43:10), “You are my witnesses . . .”

Sources:
Vayikra Rabbah 19:2; Baal Haturim and Kli Yakar to Deuteronomy 6:4.