What size water bottle is best for a preschooler vs. an elementary schooler?

Across parenting communities we read, 12oz is the most-recommended size for toddlers and preschoolers because it's light enough for small hands and fits car-seat and stroller cup holders, while 16oz is the common step-up for elementary-age kids who need more capacity for a full school day. Bigger 24–32oz bottles get praised by older kids and teens but are frequently called heavy in the same threads.