Purpose: The numbers of early childhood education programs are dramatically increasing throughout Turkey, and the regulations to ensure quality are frequently changing. A very limited number of studies have examined the quality of Turkish ECE programs on children's development. Therefore, the purpose of this study is to identify the effects of classroom size and duration on the development of children. Research Methods: This is a relational survey study that was realized with 836 children attending 55 public early childhood programs in Denizli. These children were selected through a cluster sampling method. Data were collected through the "Ankara Developmental Screening Inventory" and "Informational questionnaire." Data were analyzed using repeated ANOVAs, factorial ANOVA (2X4) and independent t-test techniques. Findings: This research revealed that there were statistically significant interactions between children's development and the structural quality components of classroom size and duration of education. Our findings showed that classrooms containing 20 to 24 children had higher developmental scores than other groups. Additionally, the results indicated that children enrolled in full-day programs scored higher on the language and cognitive development subscale and total ADSI. Implications for Research and Practice: The findings revealed that the children attending full-day programs with classroom sizes averaging 20 to 24 differed significantly from the children in other groups. This suggests that full-day programs have positive effects on children's development; therefore, it is important to increase the number of full-day programs in Turkey.