At what size does a planet cease to be described as a planet.?

The IAU defines a planet based on 3 criteria:

1. Spherical due to its own gravity

2. Orbits the sun and not another planet

3. Has cleared its orbital path of debris

If the Earth was compressed to the size of a bus, it would still fit those three criteria. A planet is not defined by its diameter, only by those 3 things.

It doesn't. There's nothing in the IAU definition of "planet" that specifies a lower limit to the radius of a planet.

The criterion are as follows: In order to be considered a "planet", an object must:

(A) Orbit a star.

(B) Have enough mass to have pulled itself into a round shape.

(C) Clear its orbital neighborhood of debris.

If you compress Earth (and I'm not sure how you'd do that), you'll increase its surface gravity, which will only improve its roundness. There's no minimum size limit, as long as it's still sphere-shaped.

I hope that helps. Good luck!

A planet is defined as an object that orbits a SUN.

But asteroids and comets ALSO orbit suns. But Asteroids are defined as being SIGNIFICANTLY small than planets.

HOW BIG has nothing to do with being as planet. It is all about HOW SMALL is an asteroid.

On the smaller side, it needs to conform to the IAS 2 criteria. On the Large size, it can't achieve fusion.