What size camping mug should I get?

450 ml (about 15 oz) is the most-recommended size across the ultralight community — big enough for a generous coffee or to rehydrate a small meal, small enough to nest a fuel canister. If you're cooking actual meals in the mug, 700–750 ml is the more flexible choice. For car camping with a dedicated cookpot, 12–16 oz insulated mugs are usually plenty.