It is customary when writing the Megila to ensure that the first word of every column is "Ha’melech." However, as the letters of the Megila should be written the same size, a Sofer should not make some letters bigger than others in order to arrange it that the word "Ha’melech" appears at the top of every column. Nowadays, most Sofrim write Megilot by copying from a Megila that has already been written with the word "Ha’melech" at the top of every column, so they know precisely how to arrange the text, and this is a perfectly acceptable method (Hazon Ovadia – Purim, p. 250).
When writing a Sefer Torah, it is proper to write at least three columns of text on each piece of parchment. (A Sefer Torah is comprised of several pieces of parchment sewn together.) When it comes to Megilat Ester, however, this is not necessary. Hacham Ovadia Yosef rules that since even a Sefer Torah is valid for use if the Sofer mistakenly wrote fewer than three columns of text on a piece parchment, this may be done even Le’chatehila (optimally) when it comes to a Megila.
It is preferable to leave the same amount of empty space around the text of every column as is required when writing a Sefer Torah. This means leaving three Esba’ot (fingers’ width) of empty space on top of the text, four Esba’ot underneath the text, and two Esba’ot in between every two columns. Nevertheless, a Megila is valid for use even if there is less empty space, as long as there is two Esba’ot of empty space on top of the text, three Esba’ot underneath the text, and one Esba in between every two columns.
Summary: It is customary to write the text of the Megila such that "Ha’melech" is the first word of every column, though the Sofer should not write some words larger than others for this purpose. Although when writing a Sefer Torah it is preferable to ensure that at least three columns of text appear on each piece of parchment, this is not necessary when writing a Megilla. It is preferable when writing a Megila to leave the same amount of empty space above, below and alongside each column of text as is required when writing a Sefer Torah.