The force of gravity between two objects is inversely proportional to the distance between the center of mass of the objects squared.
Newton's Law of Universal gravitation is:
F(g) =G m(1) m(2) / d^2
where F(g) is the force of gravity
G is a constant (6.67 X 10-11 N m^2 / kg^2)
m(1) is the mass of the first object (the earth)
m(2) is the mass of the second object (you)
d is the distance between the center of mass of the objects (distance from you to the center of the earth)
m(1), m(2), and G stay the same, but since the earth is twice as big (and thus twice the radius) and you would be standing at the surface of the earth, you would be twice as far away from the center of the earth.
since d is doubled, and F(g) is proportional to 1 / d^2:
1 / (2)^2 = 1/4
Thus gravity would be 1/4 what it was.
Since weight is simply the measure of the gravitational force on an object, your weight would be 1/4 what it was.
Mass doesn't change; however, weight does change depending on the mass of the object, the mass of the other object the first object is near, and the distance from that other object.
This is why gravity on earth is actually different for two objects of different mass. An object with a larger mass (say a semi truck) would have a larger force of gravity acting on it than a smaller mass (say a person). We just say that gravity on earth is equal to 9.8 m/s^2 for all objects because the relative difference is so small that it is not significant. It is sort of like adding a thimble full of water to an ocean. It is not really going to change the amount of water in the ocean by any measurable amount. a math and physics teacher