What size water bottle is best for a kid?

For preschool and early elementary, reviewers across r/moderatelygranolamoms and major retailers consistently recommend 10–14 oz bottles, which are light enough for small hands and fit standard lunchboxes. Older elementary kids and middle schoolers tend to do better with 16–20 oz, especially for sports days. Parents on Reddit repeatedly note that a too-heavy bottle is the fastest way for a kid to stop using it.