Unless otherwise stated, measurements are given in units as they would be defined on Earth. One day is about 24 Earth hours; one year is about 365 Earth days. Gravitational acceleration is given in g, where 1g is the average of what could be experienced on the surface of Earth.

Metric or imperial units of mass, volume, and distance are fine. Use whichever you like, though providing both is excellent. When describing extremely large objects (planets and stars, for example), feel free to give the comparison in terms of another known object (e.g. “this planet has 2.4 Earth masses”).