The exact size of the universe is still a topic of ongoing scientific research and speculation. While scientists do not currently have a definitive answer, I can provide you with some information on how the estimation you mentioned may have been made and the methods scientists use to explore the size of the universe.
The observable universe is the portion of the universe that we can observe from Earth. It is limited by the distance light has had the time to travel since the Big Bang, given the finite speed of light. The current estimate for the radius of the observable universe is about 46 billion light-years. However, due to the expansion of the universe, the region we can observe today has a larger radius than the distance that light has actually traveled.
Now, regarding the estimation you mentioned, it is important to note that without a clear understanding of the overall structure of the universe beyond the observable portion, it is difficult to determine its exact size. However, scientists have proposed various hypotheses and models based on our current understanding of cosmology.
One such hypothesis is that the universe is "globally" flat and infinite in size. In this scenario, the universe would extend far beyond the limits of our observational capabilities. However, it is also possible that the universe has a non-trivial topology, which means that it may have a specific shape or could be connected in a way that allows it to be finite while still lacking an edge.
The estimation you mentioned, that the universe could be at least 250 billion times larger than the observable universe, might be based on certain inflationary models of the early universe. Inflation is a theory that suggests the universe underwent a period of rapid expansion shortly after the Big Bang. According to some inflationary models, the universe could be much larger than the observable portion due to this early expansion.
It's important to emphasize that these estimates and hypotheses are subject to ongoing research and revision as our understanding of the universe improves. Scientists use various methods, such as analyzing the cosmic microwave background radiation, studying the large-scale distribution of galaxies, and investigating the properties of dark matter and dark energy, to gain insights into the structure and size of the universe. However, definitive answers regarding the precise size of the universe are still elusive and remain an active area of study in cosmology.