What size belt bag do I actually need for travel?

For phone, wallet, passport and a few small essentials, a 1–2L belt bag (Lululemon Everywhere, Bellroy Laneway, Pander) is plenty. If you want to carry a compact camera, a small water bottle, or layers like a packable shell, reviewers consistently point toward something in the 5–14L range like the Carhartt sling or a Waterfly-style hip pack with bottle holders.