We are interested in how cell size and organ size are developmentally determined in Arabidopsis flowers, particularly the sepals. I will discuss two collaborative projects in which modeling both helped us direct our experiments and interpret our experimental results. In examining the robust control of organ size, there was a eureka moment when we realized that our initially separate modeling and biological projects were converging on the same idea: variable cellular growth undergoes spatiotemporal averaging to create reproducibly sized and shaped organs. In this case, the modeling directed us to analyze the cellular variability and explained why reducing cellular variability increased organ shape variability. Second, in examining cell size control, model development coincided with biological experiments and the two informed each other to reveal that fluctuations of a transcription factor determine whether a cell becomes a giant or a small cell.