Which process or processes change the size and shape of the oceans?

The movement of continental "plates" across the surface of the Earth. This is a very slow process, but is accompanied by earthquakes and volcanoes.

For example, the Hawaiian Islands are all volcanic; each island is a volcano. As the Pacific plate moves northwest, a static "hot spot" in the mantle forms new islands. The Big Island is the current one, but a new submarine volcano is erupting to the southeast that will, someday, form a new island.

In the Atlantic Ocean, the North American plate is moving away from the European plate, and the Atlantic is getting wider. (VERY slowly!) The Mid Atlantic Ridge is the place where the plates are separating, and building new mountains and new seafloor.