There isn't one single unit used to measure a planet. It depends on what aspect of the planet you want to measure:

* Diameter/Radius: Measured in kilometers (km) or miles (mi).

* Mass: Measured in Earth masses (M⊕) or kilograms (kg).

* Surface area: Measured in square kilometers (km²) or square miles (mi²).

* Volume: Measured in cubic kilometers (km³) or cubic miles (mi³).

* Surface gravity: Measured in meters per second squared (m/s²).

* Orbital period: Measured in Earth years (yr) or days (d).

So, the unit you choose will depend on what aspect of the planet you are interested in.