When does the stars size and mass stabilize?

A star's size and mass stabilize during its main sequence phase, which is the longest stage in its life cycle. During this phase, the star achieves a balance between the gravitational forces pulling inward and the nuclear fusion reactions pushing outward. For most stars, this stability lasts for billions of years, until they exhaust their hydrogen fuel and begin to evolve into later stages, such as red giants or supernovae, depending on their mass.