Creating critical mass has become a cornerstorne of science policy on both national and transnational levels. It suggests that once a certain size is reached, exponential growth or accelerated change will occur. In reconstructing theories, policies and practices associated with critical mass, the paper traces the image of critical mass as a size that performs in different dimensions. It takes the German Excellence Initiative, a state programme for the competitive distribution of research funds, as a policy intended to create critical mass. That the Excellence Initiative did not trigger exponential growth but organisational change points to qualitative effects of size. On the basis of case studies, it is reconstructed how Excellence-funded graduate schools and others perform size. They accumulate prestige, build-up research capacity, reproduce 'schools of thought' or provide for research personnel. In doing so, the schools are not a critical mass for exponential growth but normalise sizing practices in higher education.