Updated project metadata. How cells control the overall size of membrane-bound organelles is an important unanswered question of cell biology. Fission yeast cells maintain a nuclear size that is proportional to cellular size, resulting in a constant ratio between the nuclear and cellular volumes (N/C ratio). To shed light on nuclear size control we conducted a genome-wide visual screen of a fission yeast gene deletion collection for mutants altered in their N/C ratio, and found that defects in both nucleocytoplasmic mRNA transport and nuclear membrane proliferation alter the N/C ratio. Perturbing nuclear mRNA export causes general bulk accumulation of both mRNA and protein, and a N/C ratio increase which is dependent on new membrane synthesis. Dysregulation of nuclear membrane growth also results in an enlarged N/C ratio, and additionally generates an aberrant nuclear shape. We propose that both properly regulated nucleocytoplasmic transport and nuclear membrane growth are central for controlling nuclear size.