Actually, Kepler is able to determine both size and mass of planet candidates, using the method of transit photometry.

For further info, I found a non-paywalled copy of Bucchave et al's Nature paper. Figure 3 plots planet radius against star metallicity, and some of the planets are clearly of Earth-radius or smaller. I very much doubt that it is possible to form gas "giants" of Earth size, and in any case they would have a mass much lower than Earth mass, so would stand out immediately.