Judging from the orbital map of Luna Nectar has a massive overall area. It appears to take up almost 1/5th of the Mare Nectaris making it ~17000 square kilometers, or a bit smaller than wales. (population ~3 million) As the population is about ten million people (I think, not sure) the city is either not very dense, or much of it's area isn't living space. As Nectar is one of the primary loaders for the Lunar Skyhook I'd expect there is a lot of warehousing around, and much of the city is supposed to be non-pressurized. I'd be a little skeptical of physical size estimations from that map though, as Shackle ends up being about the same size, but with a population of 50k. I think it would make more sense if those blue markings on the Lunar map were land held by a city rather than the city itself, but that does make it hard to judge size. For reference Seoul has about as many people, and takes up ~600 square kilometers. Sorry if that's not super helpful, but I don' know of better information to work from. Here's a fan made map and such of Nectar: http://eclipsephase.com/so-just-what-nectarluna