Chapter four of Mary Shelley's Frankenstein describes Victor's decisions regarding the proportions of his creature.
Victor spends almost two years forming the creature, and his obsessive nature can be seen through his decisiveness in choosing the parts for his creation. Not wishing to waste time collecting and examining small body parts, Victor decides to simplify his task and create a being which is rather large in nature.
Given that a large being will use larger parts than a normal man, Victor decides that to alleviate the "hindrance" to his speed, he will make the creature around eight feet tall.
As the minuteness of the parts formed a great hindrance to my speed, I resolved, contrary to my first intention, to make the being of a gigantic stature; that is to say, about eight feet in height, and proportionably large.
Therefore, after deciding that small pieces will take far too long to put together, Victor opts to create a being far larger than himself.
He decided to go with larger proportions, making a very large man. Dr. Frankenstein reasoned that the larger parts would make construction easier. This is why the monster is described as such a grotesquely large creature.