The size effect is sensitive to the length of the return interval used in estimating betas. Beta changes with the return interval because an asset's covariance with the market and the market's variance do not change proportionately as the return interval is changed. We document beta sensitivity to the return interval. Evidence from cross-sectional regressions of returns on monthly and annual betas is inconsistent with beta changes stemming only from the higher standard errors of the longer-interval betas. We provide evidence that the size effect becomes statistically insignificant when risk is measured by betas estimated using annual returns.