The potential occurrence of variable returns to size in research activity is a factor to be considered in choices about the size of research organizations and also in the planning of national research assessment exercises, so as to avoid favoring those organizations that would benefit from such occurrence. The aim of the current work is to improve on weaknesses in past inquiries concerning returns to size through application of a research productivity measurement methodology that is more accurate and robust. The method involves field-standardized measurements that are free of the typical distortions of aggregate measurement by discipline or organization. The analysis is conducted for 183 hard science fields in all 77 Italian universities (time period 2004-2008) and allows detection of potential differences by field.