Using a unique firm level data – Enterprise Surveys - we develop a new measure of credit constrained status for firms using hard data instead of perceptions data. We classify firms into 4 categories: Not Credit Constrained, Maybe Credit Constrained, Partially Credit Constrained, and Fully Credit Constrained to understand the characteristics of the firms that fall into each group. In particular we look at firm size as a potential determinant of credit constrained status. First, we find that SMEs are more likely to be credit constrained (either partially or fully) than large firms. Furthermore, they finance their working capital and investments mainly through trade credit and informal sources of finance. These two results hold to a large extent in all the regions of the developing world. Second, although size is a significant predictor of the probability of being credit constrained, firm age is not. Third, high performing firms measured by labor productivity are less likely to be credit constrained. This result applies to all firms but is not as strong for small firms as it is for large and medium firms. Finally, in countries with high private credit to GDP ratios firms are less likely to be credit constrained.