The influence of tip size on surface roughness measurements by atomic force microscopy (AFM) was evaluated experimentally. Images of quartz over a lateral scan range of 125-5000 nm were measured by AFM utilizing tips of different sizes. Tip size was quantified by transmission electron microscopy before and after AFM imaging. Only tips whose size remained relatively constant during imaging were used in this study. Two different trends were observed for measured surface roughness as a function of tip size. At small lateral scan sizes (less than or equal to 500 nm). the image root mean square roughness decreased as tip size increased. However, at larger scan sizes (e.g. 5000 nm), the roughness increased with increasing tip size. A simple two-dimensional model was developed to explain both trends.