||Aim of this paper is to make a literature review and also to empirically assess op-timal size of cooperative banks in European Union in terms of their cost efficien-cy. Theoretical explanations for possible efficiency advantages are brought for-ward and consequently they are compared with existing empirical literature. Fur-thermore, we created dataset of 283 banks from 15 European countries between years 2006 and 2013 to empirically investigate this problem on recent data. Using descriptive statistics, we found that bigger European cooperative banks are more efficient in terms of Cost-to-Income ratio than the smaller ones. These results seem to be stable in time and also in individual countries as was shown on analy-sis of sub-samples.