I recently moved into an apartment and am looking to purchase a few Mezuzot for it. Would you please tell me what is the proper size Mezuzah for the front door as well as the doorways inside the apartment?

A Mezuzah can be any size you like as long as it is Kosher. The smaller the Mezuzah is, the harder it is for the Mezuzah to be Kosher since it is hard to write such small letters and still be Kosher. One should consult with a rabbi about choosing a good Sofer to purchase the Mezuzot from. Rabbi Yitzchak Grinblat