Mezuzah scrolls come in standard sizes. It used to be that the most common size was 10 cm and some people who wanted a bigger mezuzah bought a 12-cm scroll. But in recent years sizes went up a notch: 12 cm is very common and those who want bigger move up to 15 cm.
The jumbo mezuzahs you sometimes see, especially at Chabad homes, are typically 20 cm, and the very small ones are 7 cm or even 6 cm. A 6-cm or 7-cm scroll is quite hard to write and compromises the sofer’s ability to ensure the writing is 100% kosher. So you’re often getting less for your money. Generally it’s recommended you stay away from these sizes.
Bigger may be better up to 12 cm or 15 cm, but beyond that size, it becomes harder for the sofer to do his job.
A 10-cm mezuzah is 3.9 inches, a 12-cm scroll measures 4.7 inches and a 15-cm scroll is 5.9 inches.
If I’m not mistaken, Carma Winery recently broke the record for the world’s largest mezuzah.