Mr. Fantastic, also known as Reed Richards, has the ability to stretch his body into virtually any shape and size due to his highly malleable, elastic- like body. In most Marvel comics, he can stretch up to about 1,500 feet (approximately 457 meters), which is roughly the length of five football fields. This is considered his practical upper limit for controlled stretching, as beyond this, his muscles cannot effectively support or manipulate his limbs. However, in some versions of the story, especially when aided by external cosmic powers, he has been shown to stretch for several kilometers, or even merge with machinery, indicating that his upper limit can sometimes be pushed much further or appear limitless. His body works like living rubber, allowing him to flatten, twist, expand, reshape, and absorb impacts with his elastic properties. Yet, stretching to extreme lengths repeatedly or suddenly can cause physical strain and even tearing of his skin, revealing some vulnerability. In summary, typically, Mr. Fantastic can stretch around 1,500 feet, but this distance can vary depending on the storyline and external factors enhancing his abilities.