Increased growth in early childhood has been suggested to increase the risk of type 1 diabetes. This study explored the relationship between weight, height and development of persistent islet autoimmunity and progression to type 1 diabetes during the first 4 years of life in 7,468 children at genetic risk of type 1 diabetes, followed in Finland, Germany, Sweden and US. Growth data collected every third month were used to estimate individual growth curves using mixed models. Cox proportional hazards models were used to evaluate the body size and risk of islet autoimmunity and T1D. In the overall cohort, development of islet autoimmunity (n=575) was related to weight z-scores at 12 months, (HR 1.16 per 1.14 kg in males or per 1.02 kg in females; 95%CI 1.06-1.27, p<0.001, FDR=0.008), but not at 24 or 36 months. A similar relationship was seen between weight z-scores and development of multiple islet autoantibodies (1 year, HR 1.21 95%CI 1.08-1.35, p=0.001, FDR=0.008; 2 years, HR 1.18 95% CI 1.06-1.32, p=0.004, FDR=0.02). No association was found between weight or height and type 1 diabetes (n=169). In conclusion, greater weight in the first years of life was associated with an increased risk of development of islet autoimmunity.