Svaret finns på sidan 195 i PHB:
Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack. Certain creatures (typically those larger than Medium) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
Förutom att vissa vapen också ökar din räckvidd om de har räckvidd -egenskapen, som beskrivs på sidan 147 i PHB:
Reach. This weapon adds 5 feet to your reach when you attack with it.