Yo. Physics degree in da house.

1. The force gravity will exert depends on the rider's mass.

2. Mass depends on volume, which is the cube of linear size. I.e. if you double the radius of a solid ball then it's weight goes up eight times.

3. Air resistance depends on the square of linear size. So the double size ball will have four times the air resistance of the original.

So as size increases gravitational force increases faster than air resistance, all things being equal.

However, air resistance also depends on the square of velocity - so big increases in mass equate to only small increases in speed. I'd expect that double-size, eight-times-heavier ride to go down a hill only 40% faster than Mr Standard Size at most - terminal velocity will increase with the square root of linear size (e.g. rider height.) The difference between two identically built riders of 5-10 and 6-4 will be about 4%.

The case is different for a tandem - it has double weight, but air resistance isn't doubled - the stoker is drafting behind the steerer.