I've moved into a new apartment and need to buy some mezuzot. Looking around, each seller seems to offer a range of options in terms of price. It seems like the way it works is the bigger the klaf, the higher the price, though I'm not sure. Is there any reason to choose one size over the other or to choose a more expensive klaf over a less expensive one? Aside from size, there also seem to be choices to make in terms of shitah, with terms like Ari, etc. Do these characterize issues that make a difference halakhically?
There are several opinions on the forms of the letters themselves. If your ancestry hails from Yemen, they have their own distinctive script. If you have Hassidic/Kabbalistic roots (or leanings), then Ktav Ari is optimized for you. (There is a tension in the letters that represents the tension in the universe we have to correct; e.g. the Chet is assymetric, and the right head of the Tzadi points away from the body.) If you're non-hassidic Ashkenazic or of Spanish origins, then Ktav Beit Yosef is preferable.
Generally a "more beautiful" mezuzah is considered better, and keep in mind that once you get teeny-tiny, the odds the scribe got everything right go down a bit (and things become more fragile). On the other hand, it can take a skilled scribe to do a good job even small; anyone can do a nice job when you have lots of space! Some of this may even be objective (assuming all are kosher and durable)-- look over the parchments, what do you consider more beautiful?
I would add to Shalom's answer that the standard sizes are 10, 12, or 15 centimeters. As a sofer, albeit with limited experience, my opinion is that the best balance between quality and price is found in 12 centimeter mezuzot.