All hexes immediately adjacent to a combatant's hex are subject to attack. Therefore, there is a larger circle over which a combatant can affect enemies. This is called melee.
All movement in combat is measured against a combatants ability to move from one hex to another. Details about this can be obtained in the rules for Movement in Combat. It normally requires 1 action point (AP) to move from one combat hex to another.
Naval hexes, for determining the movement of ships on the water, have a diameter of 20 ft.