A resposta pode ser encontrada na página 195 do 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.
Além disso, certas armas também aumentam seu alcance se tiverem a propriedade reach , conforme descrito na página 147 do PHB:
Reach. This weapon adds 5 feet to your reach when you attack with it.