Quoting Simon O'Connor
The GT40 is so named because it was only 40 inches high. The scale is about right, as far as anything is in scale with a minifig. The only thing that looks off to me are the wheels. They should be a bit bigger in proportion to the body. Not sure if it would be easy to fix though.
These wheels do look small, but there's no way to fix it for now, as they're the most suitable for a car in this scale.