It has recently been shown that a three-parameter Weibull function with a large threshold strength of approximate to 2GPa is needed to accurately describe the failure strength statistics of micromachined polycrystalline silicon samples. Here, we explore how to apply this function to predict strength size effects over a size range of 100. A two-parameter function is unsatisfactory in predicting the size effect. If a three-parameter Weibull fit to only the largest specimen is used, the prediction also does not satisfactorily agree with strength data in smaller specimens. The prediction is greatly improved if the two largest specimens, a factor of 10 different in size, are used for fitting. It is further demonstrated that the threshold strength depends on geometry in notched samples due to their large stress gradients.