Abstract While the importance of the refuge structure for coral spat survival is increasingly recognized, little information is available regarding its effective size or dimensions. Accordingly, the present study examined the effective size of refugia using spats of two reef coral species, Echinophyllia aspera and Favites abdita. Coral spats settling on plates with three sizes of refugia (5, 15, and 25mm wide, and all 4mm deep) were monitored over a 2-year experimental period. The results were similar for both coral species: a higher survival rate was found for coral spats in smaller refugia. The 2-year survivorship in the 5-mm refugia (E. aspera: 1.5%, F. abdita: 12%) was more than one order of magnitude higher than those in the two larger refugia. Considered together with previous information, the effective size of refugia for the survival of coral spats may be on the scale of millimeters.