What size water bottle is best for hiking?

32 oz (roughly 1L) is the consensus standard across the reviewers we read — it lines up with most water filter threads, fits side pockets on virtually every pack, and is the size most hydration math (1L per 2 hours of moderate hiking) is built around. 48 oz / half-gallon makes sense for desert or dry-camp use; smaller insulated 24 oz bottles are popular for short day hikes.