In terms of art, it can refer to the physical proportions of the body. The average human being is 7 heads tall (the height of seven heads). Heroic proportions would describe someone who is 8 to 8.5 heads tall...as depicted in Greek sculptures.
'Of heroic proportions' refers to something that is of huge or gargantuan magnitude. It refers to that act of heroism that is larger that life, extraordinary. It may also refer to something that has exceeded normal expectations in terms of its scope. For example, 'the plane achieved a speed of heroic proportions'. For example, "the English cricket team managed to make a score of heroic proportions".