Barley (Hordeum vulgare L.) ranks as the fourth most cultivated cereal crop globally by planting area. Kernel characteristics, including grain length, grain width, and thousand-grain weight (TGW), are essential determinants of barley yield and quality. The identification and cloning of genes related to kernel traits, along with the detection of superior alleles, are fundamental for marker-assisted selection in barley breeding. This study presents the cloning of HvGL7-2H from barley, based on the known rice GL7 gene. The functional significance of HvGL7-2H in grain length was confirmed through ethyl methane sulfonate (EMS) mutants of the barley landrace “Hatiexi”. A candidate gene-based association analysis was conducted using a panel of 363 barley accessions to identify superior haplotypes for HvGL7-2H. The analysis revealed that Hap3 represented the superior haplotype for both grain length and TGW, while Hap4 emerged as the superior haplotype for TGW. These findings indicate that genotypes carrying the superior allele serve as valuable genetic resources, and the molecular markers identified herein will facilitate grain size and yield improvement in barley breeding programs.