Grain size and weight are two of the most important determinants of crop yield. Key genes associated with the grain size and weight have been identified in major crops. However, studies on the genetic basis of the grain size and weight related genes in wild Sorghum are limited. In this study, we analysed the variation of grain size related genes using variant analysis of 15 accessions across one cultivated and six tertiary gene pool species representing the five subgenera of Sorghum. A wide variation in grain size related parameters was observed. The highest grain weight, width, and thickness was observed for the accession S. bicolor (L.) Moench 314,746, while the highest grain length was observed for the accession S. macrospermum E.D. Garber 302,367. The wild sorghum species exhibited high morphological diversity. The six candidate genes related to grain size, Sobic.001G335800 (qGW7/GL7), Sobic.001G341700 (GS3), Sobic.002G257900 (GW8), Sobic.003G035400 (GW5/qSW5), Sobic.004G107300 (GW2), and Sobic.009G053600 (GS5) showed polymorphism in the coding sequence regions including variants generating premature stop codons. These variants might contribute to the observed variation in grain size and weight. The tertiary wild sorghum species may be a useful source of genes for understanding and engineering grain size in sorghum and other cereals.