I apologize for the confusion. UY Scuti was indeed considered one of the largest known stars. However, scientific knowledge and discoveries are constantly evolving, and new information may have emerged since then. If there is a star named Stephenson 2-18 that has been identified as larger than UY Scuti, it would be a more recent discovery.
It's important to note that determining the exact size of stars can be challenging due to various factors such as distance, variability, and the complex nature of stellar atmospheres. Different measurement techniques and observational data may lead to varying estimates of stellar sizes.
To stay up to date with the latest scientific findings and confirm the current status of Stephenson 2-18 in relation to UY Scuti, I recommend consulting recent scientific publications or resources from reputable astronomical organizations and observatories. They will provide the most accurate and current information on the topic.