These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. Large tornadoes can be weak, and small tornadoes can be violent. Another consideration is the stage in the life cycle of the tornado.
Large tornadoes can also be strong and small tornadoes can be weak.
The Fujita Scale is based on damage, not the appearance of the funnel. Storm spotters, storm chasers and other weather observers often try to estimate the intensity of a tornado when they are in the field, basing their judgement on the rotational speed and amount of debris being generated as well as the width. However, the official estimate is made after the tornado has passed. Personnel from the National Weather Service office that issued the warning survey the site to determine the F-Scale rating. Sometimes they call in experts from out of the area.