The concept of the "size" or "radius" of the universe is a bit more complicated due to the expansion of space. The 18 billion light-year distance you mentioned refers to the distance that light from a galaxy 18 billion light-years away has traveled to reach us since the Big Bang. However, during the time that light was traveling, the universe has been expanding, so the current distance to that galaxy is much greater.
The expansion of the universe means that the space between galaxies is stretching, causing them to move apart. It is important to note that this expansion happens on the scale of the entire universe and is not limited to the galaxies themselves. Therefore, while the light from a galaxy 18 billion light-years away has traveled that distance to reach us, the current distance to that galaxy is actually much larger due to the expansion of space.
To estimate the current distance to a galaxy, cosmologists use a measure called "comoving distance." Comoving distance takes into account the expansion of the universe and provides a more accurate representation of the current separation between objects. While the comoving distance to a galaxy 18 billion light-years away may be difficult to determine without additional information, it would indeed be much larger than 18 billion light-years.
The size of the observable universe, which is the part of the universe we can currently see, is estimated to be about 93 billion light-years in diameter. However, it's important to note that this size refers to the observable universe at the present time and does not imply a physical size or radius of the entire universe, which could be much larger.
In summary, while light from a galaxy 18 billion light-years away has traveled that distance to reach us, the current distance to that galaxy is much greater due to the expansion of the universe. The overall size or radius of the universe is a complex topic and is still a subject of ongoing research in cosmology.