An article from Healthbolt raised couple of good points on your feet and how to buy shoes that fit. The post talks about your feet size will change throughout the entire life, and what are the actual ways to measure them and get the correct size of shoe:
Measure your feet once a year. Your foot size will change throughout your entire life. It may not be as dramatic as the 18 and under crowd, but they do change. For instance: when I started running about 2 years ago I was a 10.5; now I’m an 11. (I’m 25 years old.)
There are three dimensions you should measure your feet by when purchasing a shoe. Size (which is really “length”), arch and width.
All three of these can be measured within a few seconds by anyone at a reputable shoe store. Make sure they measure both feet since no one’s feet are both the exact same size…
Buy Shoes that Fit Every Time – [Healthbolt]