How large does it have to be to a Vulcanoid asteroid?

I do not believe there is an official minimum size for a minor planet. I have seen NEO minor planets as small as 10m in diameter.

To me a speck of dust should not gain the status of minor planet, but I do not believe that the IAU has set forth minimum size guidelines to prevent the speck from being a minor planet. The only think that prevents tiny objects from being minor planets is our ability to detect them and establish their orbit.

The NASA NEO Potentially Hazardous Asteroids list limits their concern to objects with an absolute magnitude of ≥ +22.0.  When we speak of absolute magnitude with respect to solar system objects, we use the absolute magnitude for planets definition.  According to the NEO absolute magnitude table, this would imply a minimum diameter of 110 meters.

The NEO Near Earth Asteroids table of orbits includes objects that have much fainter absolute magnitudes.  As of today, that table includes objects with an absolute magnitude as faint as 28.8 and a diameter as small as 10 meters.

An Vulcanoid 1km in diameter might have an absolute magnitude as faint as +17.  At 100 meters in diameter, Vulcanoids might have an absolute magnitude as faint as +22.  At 10 meters in diameter, Vulcanoids might have an absolute magnitude as faint as +27.

When an object gets small enough, it will have a difficult time staying inside the stable Vulcanoid zone for very long (multiple GYr). 

chongo (Landon Curt Noll - http://www.isthe.com/astro.html) /\oo/\