planet out of dust

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If you could magically build a planet out of dust in the space of our solar system, at what point (how large) would the material used go into orbit around the sun?

If you made a ball of anything in space, and it wasn't in some type of orbital motion already, it would eventually just fall into the sun (or collide with or get captured by another celestial body within the solar system). It would not start orbiting all by itself just cos it reached a certain mass.

Look up how solar systems are formed.

1) the dust is already in orbit around the parent star as it is coalescing into a protoplanet.

2)Its not a matter of "how big". Its how fast. Anything slower than the escape velocity from the gravitational pull of the star (at that particular distance from the star) falls into the star. Anything faster escapes the stars pull and goes off into space. At just the right speed it goes into orbit around the star. Matter orbits the sun in all size packages- from specks of dust, to boulder sized asteroids, comets, up to the gas giant Jupiter (eleven times as wide as the Earth).