PURPOSE: Family size is linked tightly with children's language development. Yet, we have a limited understanding of the associations between family size, specifically the number of adults and siblings, and children's social communication skills, particularly in children from diverse linguistic environments. The present study examined the effects of family size on social communication abilities in 4- to 5-year-old monolingual and bilingual children. METHOD: Parents of 86 English-speaking monolingual children and English-Spanish-speaking bilingual children aged 4-5 years completed the Children's Communication Checklist-Second Edition (CCC-2) to measure the children's social communication skills. Individual subscales covering the pragmatic language aspects (or, for brevity, "pragmatic subscale(s)") in the CCC-2 were considered as outcome variables. Parents also completed interviews where information was obtained regarding the number of family members, including adults and siblings, in the household. RESULTS: Results revealed that the number of adults did not predict children's social communication skills. However, the total number of family members and the number of siblings significantly predicted aspects of children's social communication abilities. Results showed that a smaller family size and fewer number of siblings may be more beneficial to monolingual children, whereas a larger family size and a greater number of siblings may be more favorable to bilingual children. CONCLUSIONS: Findings from this study provide early insights into how family-related structures can impact the development of social communication skills in children from varied linguistic backgrounds. That is, family composition differentially affects monolingual and bilingual children's social communication, and they do so for different aspects of pragmatics.