The first working map that J.R.R. Tolkien made of Middle-earth shows Rhosgobel situated on the edge of Mirkwood just south of the Gladden Fields. However, a note appended to the essay on the Istari in Unfinished Tales places Rhosgobel farther north, between the Old Forest Road and the Carrock.
http://www.tuckborough.net/dwellings.html
I found this looking for information. I think that Tolkien wasn't set on where it was and had changed it. That said, I always believed Radagast lived in the woods near by Mirkwood, but did not interact much with Elves or Humans but with animals and plants, so he would not be out and about within the normal paths and Rhosgobel could cover the area between southern edge of mirkwood and going further north between the Old Forest Road ect.