When scientists began studying the nature of these mysterious asteroids for the very first time, it was not an easy task given the absolute lack of any observatory details. So they went ahead and made the following postulations logically.
Given their close proximity to the sun, any celestial object with diameter less than a hundred meters would end up getting sucked into the sun, owing to the gravitational effect, coupled with the gigantic radiation pressure of the sun. So if vulcanoid asteroids really exist (which has to be true, as they seem to be the only plausible explanation for Mercury’s orbital calculations), then their diameter cannot be anything less than a hundred meters for sure!
The reason why the maximum diameter of vulcanoids has been estimated to be 6 kilometers would be because any celestial body in Mercury’s orbit (and that close to the sun of course) would heat up and appear red, glowing hot.
In this regard, another theory that has come up about vulcanoid asteroids is the fact these are fragments that were dismembered out of the planet Mercury, during the early stages of the solar system. The presence of ferrous metals could be another reason why the asteroids are stable and do not glow. The reason is that ferrous metals melt at a much higher temperature as compared to non ferrous elements, and if vulcanoids were made of ferrous elements, these glowing bodies that close to the sun would be aflame and very much visible.
More than their physical nature, the efforts made towards spotting or observation of at least one vulcanoid has more to do with discovering important aspects of inter-related data. For instance, studying the actual shape, size, structure and composition of vulcanoid asteroids will open up fascinating aspects of how the solar system came into being. Scientists are keen on knowing facts about the formation of the first planets as well as the early arrangement of the solar system, and in the theories that back the current hypothesis, the existence, observation and systematic study of the solar system will add a great deal of credibility to many research studies that are waiting for evidence in this area.