The diameter of a planet is the distance from one side to another through it's center. Jupiter has the largest diameter and is about 11 times that of earth.
An increace in diameter means a big increace in volume and mass. This is because the volume of a sphere is measured as volume= (4/3) * pi * r ^ 3 where r is the radius of the sphere. The radius is half of the diameter. The radius is the distance from the center of a sphere to is't outside.