Final answer:
The Gravitational force of the Moon has the greatest impact on tidal sizes, though the Sun's gravity also significantly affects tides, causing variations such as spring and neap tides based on the alignment of the Earth, Moon, and Sun.
Step-by-step explanation:
The factor that has the greatest effect on the size of the ocean tides is the Gravitational force of the Moon. Tides are primarily a result of the Moon's gravitational pull causing ocean water to rise and fall as Earth rotates through the tidal bulges that are formed.
The gravitational force of the Sun also affects tides significantly, contributing to the variation in tidal sizes with the largest tides, known as spring tides, occurring when the Earth, Moon, and Sun are aligned, and the smallest tides, called neap tides, occurring when the Sun is at a 90° angle to the Earth-Moon alignment.