How does the size of a planetary nebula evolve after the initial ejection phase?

It visibly grows larger over thousands of years as the gas spreads outward into the interstellar medium

The scale associated with a planetary nebula is not fixed immediately after formation; rather, it is a dynamic, evolving structure. Following the shedding of outer layers by the dying star, the ejected material continues its outward momentum, causing the nebula's diameter to increase gradually over extensive timescales. Specifically, this expansion occurs over thousands of years as the gas diffuses into the surrounding interstellar medium. Eventually, this material becomes so sparse that it is no longer observable as a distinct nebula, effectively blending into the background gas.