The size of the earth is just right. If it were any bigger, it's would be greater, which may have retained a hydrogen-filled atmosphere rather than the oxygen-rich atmosphere we have today.

Gravitational pull, because the more mass the planet has the more it will pull other objects to itself. An example of this is how the Earth is able to have a moon in it's orbit because it's mass is greater than that of the moon causing the moon to be pulled to stay around the earth.

The answer is; TRUE

The larger planets have a stronger gravitational pull than smaller planets. Therefore, these larger planets are able to attract and maintain even lighter gases such as hydrogen in their atmosphere. Smaller planets are able to maintain only heavier gases (such as oxygen) in their atmosphere. An example is Jupiter who's larger mass and gravitation pull is able to maintain an atmosphere of hydrogen and helium while planets like earth, venus, earth are not.