The correct statement about gravity is:
Gravity is a force that attracts objects with mass toward each other.
The other statements are incorrect or misleading. Gravity does vary with mass, but it does not vary with the "size" of an object in a way that affects the gravitational force; rather, it is the mass that matters. The statement "Gravity is a measure of an object’s matter" is also inaccurate, as gravity itself is not a measure but rather an interaction between masses.