The Gemara in Megillah states that the names of the ten sons of Haman are written in the Megillah scroll in a special configuration of "Shira"-poetry. As opposed to the regular passages in the Torah which are written line after line, passages of "Shira" are written with spaces. In the "Shirat HaYam" (song of the Sea), the verses are written so that the alternating lines have the phrases in different positions, text over space. The first line has two phrases, one at the beginning and one at the end of the line, with space in the middle. The next line will only have a phrase in the middle, with no text on either end. Like this:
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The layout of the " Shira" of the tens sons of Haman is different. The text is arranged in columns at the two ends of the lines. Like this:
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The reason for this difference is that the lines of text are analogous to bricks, which are unstable when laid in a wall one on top of the other. This alludes to the instability of our enemies and their ultimate downfall. On the other hand, the "Shira" of the "Shirat HaYam" is structured in an alternating fashion which is the proper way to build a stable structure.
The Talmud Yerushalmi in Masechet Megillah states that if the Megillah was not written with this configuration, the Megillah is Pasul (invalid). The Yerushalmi continues to discuss the proper layout of the ten sons. Although there are differing interpretations of that Yerushalmi, the Halacha is in accordance with the understanding of the Levush (R. Mordechai Yoffe, 1530-1612, Eastern Europe). He rules that on the top right corner of that page, the word "Ish"" (men) is written; on the bottom left corner, across from the last name of Haman's last son, is written the word "Aseret" (ten). This total eleven lines of text on that page. Nothing else should be written on that page, before or after this text. Therefore, the letters of that page will be of a relatively larger size so that they properly fill the space. Hacham Ovadia cites the Vilna Gaon (Rav Eliyahu of Vilna, 1720-1797) who presents a Hidur (enhancement) to write the entire Megillah with eleven lines of text. If every page is eleven lines, the size of the letter can remain consistent, whereas if the rest of the Megillah has more lines, the letters on those pages will be smaller than the letters on the page of the ten sons. This is a Hidur, but a Megillah in which the rest of the pages had more lines is also Kosher.
There is another Hidur to configure the text of the Megillah so that the top of each page begins with the word "HaMelech" (The King). Originally, the Poskim were not in favor of this practice, since to achieve that effect, the rest of the text had to be adjusted accordingly, which led to unnaturally elongating and stretching certain letters. This led to a loss of symmetry on the page. However, today, most Sofrim copy the layout from a pre-existing template in which the symmetry is preserved. Hacham Ovadia rules that in such a case a Megillah of "HaMelech" is preferred.
Finally, the first letter, "Vav" of the last son, "Vayzata," is written with a "Vav Rabati"-an elongated form. Some say that it should also be pronounced in an elongated fashion.
SUMMARY
It is preferred to use a Megillat that all its pages are written with eleven lines, and each page begins with the word "Hamelech."
