Inspired by Wavemans recent post, I decided to quantify my Headcanon assumptions for TOS. I guess that since GR ruled that everything prior to TNG is not canon, makes my life easier, though!
My primary sources are TOS (especially the first season) and the FASA game background, a lot of which came from the original Spaceflight Chronology.
1) Since the UFP is only about 130 years old by Kirks time, and warp speeds above 4 have only been possible for the last 30 or so, the UFP can't be that big. Using the FASA sourcebook map, probably an Inner Federation about 10 parsecs across, with an Outer one only a few to six parsecs beyond that.
2) Going by the order of encounter and given the above speed limitations, the RNZ must be within a few parsecs of a Terra, with the Orion's within ten parsecs and the Klingons a bit further out.
3) StarFleet is a Humans first club, simply because the other founding races simply have other concerns than exploring the universe.
4) The Constitution class was the first warp 6/8 ship, and designed specifically as the new explorer/capital ship to expand the borders of the UFP. As such, it is exceptional and it's captain and crew the elite of StarFleet. There are other single role smaller chips, but the "Starship" class are the most important.
There are other thoughts thatI don't have time to type out just now, and other posters thought welcome