How and why has our estimate of the size of the universe changed in the past few centuries?
We understand it much better now because of improved observation abilities and new theories that were developed to account for those observations.
“Centuries” is really a long time, especially in terms of recent scientific advancements! The change in human understanding from an earth-centered small sphere of celestial bodies to the unknown frontier of the expanding universe is entirely due to the ability to observe more of our universe.
People are usually “ignorant”, not “stupid”. We sometimes look back at old conceptions and laugh at them. But really, they were the best and very good descriptions of the observable phenomena. As new knowledge was obtained through better optics, and then electronics, the human observers were eager to change their theories whenever better ones were proposed. The social and political opposition was for a different reason, as it is today.
We also know that we still do NOT know exactly what the size, shape, or even movement of the universe is. Maybe the most significant scientific advancement in history was the ability to recognize that “The Science is NEVER ‘settled’ “! If you hear that it is – it is not a real scientist talking.