All I know about is Picard stating that Federation worlds are spread across 8000ly
These quote from Picard can only be used to detrmine the maximum size of the federation. Kinda like a flatten ball with a diameter of 4000ly and a hight of 500ly.
People usually assume Federation can be approximated by a solid chunk of space with dimensions of that order of magnitude but there really is no justification for that
We know there are static borders with the Klingons, Romulans, Cardasians, Sheliak there was also a colony destroyed by the Gorn, indicating some kind of border,... So there is basicly some kind of devision of space and that there must be some big solid piece of federation.
Cayenne is 7000km away from Paris yet if we assumed that France can be approximated as being by a 7000kmx5000km square we'd reach a conclusion that France is two times larger than Russia.
Lets try a differen example. Take the USA. What if they decide that all of the ocean among Alaska, Hawaii and California becomes theres. How much of the planet surface will they have.
An even more extreme example would be Marshall Islands which stretch across 1000km which could lead us to assume their total surface area is on the order of million square kilometers when in reality it's merely 181km2.
That is the best example how we should look at the federation. Each island representing a system and the sea among the islands representing the space. Of course here on Earth we have international waters that start at some distance from the coast, but in Trek we have never heard of anything like that.
