The China-U.S. trade frictions and COVID-19 pandemic have caused unprecedentedly high economic policy uncertainty since 2017. To resist this high uncertainty, investors may prefer large stocks over small stocks, thereby damaging the size effect. To test this inference, we apply data from China to show that the size effect becomes insignificant after 2017. However, a significant size effect re-emerges among stocks with low valuations or low volatility, and this is positively associated with the increment of the economic policy uncertainty index. We also find that when uncertainty increases, institutional investors increase their holdings in small stocks before 2017, but hold more large stocks after 2017. Our findings consistently suggest that high policy uncertainty may change investors' preferences for firm size and cause the disappearance of the size effect, and only among stocks with low risk, size effects may show up due to low-risk small firms' similar function in resisting market uncertainty as large firms. Other mechanisms, such as the quality premium, unexpected profitability shock, shell value, or M&A option value, are not applicable in explaining the findings in China. Our study contributes to proposing a new mechanism for the time-variability of the size effect.