The earth was formed from gas and dust, pulled together by gravity. In its early days, it may have gained or lost some size due to collisions with other planets or asteroids (people think this is how the moon formed!). But evidence suggests that since then, the size hasn't changed very much. This is a great question because until recently scientists believed the earth might still be expanding or contracting.

In 2011, some scientists at NASA used a combination of lasers, GPS, and other techniques to carefully measure the earth. They determined that the earth's size was not changing (or at least, was changing by an amount too small to detect- less than the width of a human hair every year). Of course, motion of tectonic plates means that features on the surface of the earth might change (some mountains get higher every year) but on average, the planet as a whole does not appear to be changing size.

Source: From NASA.