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Grain shape improvement, which determines grain yield, quality traits and commercial value, is an extremely important aspect of rice breeding. Grain size is controlled by multiple genes, and Maker Assistant Selection (MAS) breeding is effective for breeders in developing stable and efficient markers to aggregate these genes in order to speed up the selection of new lines with desirable traits during the breeding process. In this study, functional markers were developed based on the sequence differences of five grain-shaped genes (<i>GL7</i>, <i>GW6a</i>, <i>GS6</i>, <i>GW5</i> and <i>TGW6</i>) between the long-grain <i>japonica</i> rice variety Zhendao and the <i>indica-japonica</i> restorer R2027. We then constructed a population of recombinant inbred lines (RILs) based on their cross. The newly designed functional markers were used to genotype grain-size genes, and a genetic effect analysis was conducted to screen high-quality long-grain restorers. Our results reveal diverse effects of different genes on grain size, and the five genotypes were distributed in the 36 selected BC₁F₈ lines. Specifically, <i>gw5</i> positively regulates grain width and 1000-grain weight, <i>gl7</i> and <i>gs6</i> positively regulate grain length but negatively regulate grain width and 1000-grain weight, <i>tgw6</i> positively regulates grain length and <i>gw6a</i> positively regulates 1000-grain weight. The most outstanding outcome is that 5 of the 36 lines achieved in this study showing an excellent performance of long grain and yield characters are ideal materials not only for studying the interaction and genetic effects between polygenes but also as restorers or donors for dominant genes in <i>indica</i>-<i>japonica</i> hybrid rice breeding.