As long the Mezuzah fits in without being squashed or folded, it should be okay. So take care before buying a Mezuzah case to make sure that the Mezuzah can actually fit inside. I also must point out that Mezuzos can actually lose quality over time. Even a kosher Mezuzah that was in excellent condition and quality when created can spoil over time. For example, when a Mezuzah gets old, it often starts to dry up.
Cracks often develop in the letters, which invalidates it.
Letters can also fade, and thereby invalidate the Mezuzah or tefillin. Sometimes a slightly faded letter can be rewritten, but only a properly trained sofer knows when, so we must always bring it to a sofer to be checked. Jewish Law states that a Mezuzah must be checked about once in every three or four years. The Requirement of Mezuzah Any Jew, man or woman, who lives in an apartment or house is required to place Mezuzos on the doorposts of his or her home. It is not the obligation of the landlord or landlady.