The purpose of this study was to analyze and compare the alternative conceptions of high school students and preservice teachers on the concept of atomic size. The Atomic Size Diagnostic Instrument was developed; it is composed of eight, two-tier multiple-choice items. The results of the study showed that as a whole 56.2% of preservice teachers and 59.4% of high school students correctly answered six or more questions out of eight. New alternative conceptions about atomic size, which could be grouped under three main considerations, were found. These alternative conceptions were related to the number of protons, ionic charge, and period and group number.