Regulation of cell and organ sizes is fundamental for all organisms, but its molecular basis is not fully understood. Here we performed a gain-of-function screen and identified larp4B whose overexpression reduces cell and organ sizes in Drosophila melanogaster. Larp4B is a member of La-related proteins (LARPs) containing an LA motif and an adjacent RNA recognition motif (RRM), and play diverse roles in RNA metabolism. However, the function of Larp4B has remained poorly characterized. We generated transgenic flies overexpressing wild-type Larp4B or a deletion variant lacking the LA and RRM domains, and demonstrated that the RNA-binding domains are essential for Larp4B to reduce cell and organ sizes. We found that the larp4B-induced phenotype was suppressed by dMyc overexpression, which promotes cell growth and survival. Furthermore, overexpression of larp4B decreased dMyc protein levels, whereas its loss-of-function mutation had an opposite effect. Our results suggest that Larp4B is a negative regulator of dMyc.