When we see a star it looks much bigger in diameter to us than it really is, this picture (extracted from here) explains what I mean:
Notice that the point we see in the skynight, represented by the yellow outer circle, would actually encompass even planets orbiting the actual star, represented by the black dot in the center. In this case we see one planet inside the white circle.
My question is, can the effect of distance and brightness be measured as to know how far would we must be to watch a star that encompasses neptune?, the earth? or does it depend on different factors?