Radiographs are an important diagnostic tool available in wildlife hospitals to evaluate the size of the avian heart. Despite the large variety wild birds in the Iberian peninsula, clinical studies addressing these species are lacking. To establish reference values for cardiac size in the Bonelli's eagle (Aquila fasciata), ventrodorsal radiographs of 20 healthy birds were obtained, and the width of the cardiac silhouette, sternum, thorax, coracoid, and hepatic silhouette were measured. The ratios between cardiac width and other mentioned indices were calculated. Results showed that cardiac silhouette width should occupy 81%–93% of sternal width, 48%–57% of thoracic width, and 506%–673% of coracoid width. The cardiac silhouette width was strongly correlated with sternal and thoracic widths. A moderate correlation was found between the width of the heart and the width of the coracoid. No significant correlation was found between width of the cardiac silhouette and the hepatic silhouette. These results support that sternal and thoracic widths should be used preferentially when evaluating the width of the cardiac silhouette in this species. The values obtained in this study can be used as a reference for normal cardiac size when evaluating radiographs of Bonelli's eagles.