Here's a breakdown:
* Plate tectonics: This is a key driver of the Earth's surface changes, but it doesn't directly control the Earth's overall size. Plate tectonics involves the movement of the Earth's lithosphere (the rigid outer layer) and the interplay of these plates (convergence, divergence, and transform boundaries) creates mountains, volcanoes, and earthquakes.
* Gravity: This force is crucial because it acts on the Earth's internal mass, pulling everything towards the center.
* Internal Pressure: The immense pressure within the Earth's core and mantle counteracts the force of gravity. This pressure is generated by the heat from the Earth's interior, which causes the mantle to convect.
These forces are in a dynamic equilibrium. The Earth's size is not static but has been shrinking very slowly over billions of years due to cooling and solidification of the core. However, the rate of shrinkage is negligible compared to the Earth's overall size.
So, the Earth's size is not "kept the same" by two specific tectonic forces but by the complex interaction of forces within the Earth's interior.
