There are many types of Megillot. Nowadays most are called “Hamelech” (The king) , where every column begins with the word “Hamelech”. In addition, there are megillot “Vavei HaAmudim” (the letter vav columns), where every column begins with the letter vav – similar to the Torah scroll. There are Yemenite versions that do not fall in either category, but since these versions have no source, there are Rabbis that have come out against them.
In addition, there are different sizes. The size of the Megilla varies due to the different row heights. The most common are 21 rows and 28 rows. Also, there are sofrim who writes in 42 rows (Lubavich Chassidim only use Megillot with 42 rows), and Megillot with 11 rows (according to Rav Ovadia Yosef, the 11 row are more enhanced then the others, because the letters of the names of Haman's sons are not larger than the others).
It is important to note that when there are fewer rows in a column, there are more columns. An 11 row is very short but very long (close to 40 columns) and it is hard to spread it out when reading, as it is customary.
The acceptable height for every row is usually 8 millimeters or 10 millimeters.
I usually recommend a 28 row Megilla, with an 8 millimeter row height. If the Megilla has 21 rows, the height of the rows may be 10 millimeters.
The price is 3,500 NIS.