An author can make his/her character as big as they want.
But for plots set in the 20th and 21st centuries, for most purposes getting over 6'3" and 270 pounds is going to hem in the story a lot. Being over 6'1" is an awkward inconvenience in most parts of North America, let alone other parts of the world.
Also being too much bigger than any opponent eliminates any suspense over who will win the fights.
My favorite size is 5'0" to 5'10" in the bikini model to physique competitor size range. Someone really strong and athletic, reflexes and endurance, but can hide it under a baggy sweater and loose jeans. Also brainy.
Variety is the spice of life.