Camera body design affects the user in a couple of ways. First, the size of the body can have a major impact on comfort when being held and used. Small hands will have difficulty with large bodies and, conversely, large hands will have difficulty with small bodies. Before purchasing a camera, it’s a good idea to hold it and take a few pictures so you know if you’ll find it comfortable to use with regularity.
Size often impacts the location of buttons, dials, and other parts of the hardware you’ll need to touch and press to operate your camera. The positioning on small point-and-shoot cameras tends to be fairly simple, because there are fewer hardware controls, but the moment you step up to a smaller DSLR (such as Canon’s Rebel series) that number increases significantly. On higher-end DSLRs, the extra space tends to ensure your hands will always be able to reach and easily access the most important controls. This is a generalization, however, and you’ll want to test them out for yourself. When you do, adjust camera settings and see what all the buttons do in manual mode (so you’re aware of their full capabilities). If it feels uncomfortable or awkward to make adjustments you’ll make often, you may want to consider a different model.
While most cameras are fairly similar, the little differences in body design can have a significant impact on their ease of use. While you can generally judge a camera’s abilities without ever using it, you’ll need to test it out yourself to make sure it feels right.