Default units of proj are meters (you even specify it in the command).
The referential ellipsoid you're using is WGS84, which has mean radius 6356752.3142 meters (see proj -le
)
You can scale it down by this factor to get "unit ellipsoid":
$ proj +proj=robin +ellps=WGS84 -m 1:6356752.3142
Even easier, but not so precise, would be projecting using the unit sphere:
$ proj +proj=robin +a=1
The answer to second part of your question is outlined in this post.