If the Earth's radius was half as large as it currently is but with the same mass, how would the acceleration of gravity at the earth's surface change, and by how much

The acceleration of gravity depends on two things.

One thing is the planet's mass, so that wouldn't cause it to change.

The other thing is the square of your distance from the planet's center.

You said that the distance is reduced to 1/2 of what it used to be, so

that tells us that the acceleration of gravity ... and the weight of every

thing and everybody on Earth ... would increase by 2² = 4 times.