Humans evolved in ancestral environments as semimigratory omnivores, which has embedded several things into our instinctive sense of wellness, aka "all is alright with the world", among them a preference of living close to water, rolling hills with lots of green and sparse tree cover, an average tendency to exhibit weaknesses for certain foodgroups (such as meat and fructose), and experience cognitive biases and limitations generally steering them towards living in groups of 50-200 people. Let's assume for now these traits are preserved, i.e. we are dealing w/ Homo Sapiens Sapiens as we know and love it.
Now, I want humans in space. Now, I want A LOT of humans in space. I want humans to infect the solar system and nearby galactic regions like a plague, like an unstoppable ever expanding Zerg creep. Now, complicating things, I'm also a firm believer that human lives ought to be happy as far as possible. Thus, each human in space will need a certain amount of mass to breathe, swim, play, socially interact, have sex and procreate, and raise disgustingly happy children in.
Let's say we choose to build space habitats large enough to house a billion people each. Let's ignore to the extent possible things like the specific shape, rotational speed to maintain a livable gravity-like experience and so forth. Let us not focus of the need for ways to make up for mass loss to space for various reasons. How much mass, and how much mass per capita should I expect each of these habitats to take?
Ah, but the question is now a bit broad. Let there be constraints. So, constraints. Humans need (for each of the following, you may assume that a technical infrastructure to support them is needed, with --and I am probably guessing wrong-- a 90%:10% mass ratio between infrastructure and the thing being supported):
- day-night cycle,
- space and mass for lakes and rivers,
- space and mass for trees so a layer of soil of at least 10 feet is probably required,
- space and mass for rolling hills to wander around in,
- space and mass for cute furry/scaly/feathery things to pet and/or eat (aka a biosphere),
- space and mass for habitats to live in (caves, homes whatever)
- lots of electricity, enough connectivity to maintain a rich verbal and cultural heritage (these folks live in the future, and are not primitive, although we would probably find them strange)
- mechanical servants and tools to do some of the heavy work occasionally
Humans also like to live in communities, let's say the aforementioned villages of 50-200 people, and assume about 3 miles (5km) or so between villages.
Let's keep in mind (for the purpose of realism) that some small fraction might prefer solitude (so higher mass requirements) and that a significant fraction (5-99%) might actually prefer to live in denser communities of millions or billions of people, which means our village estimate should be a rather conservative (in the sense of being generous & expansive) estimate of the amount of mass needed.
And yes, I know that being as a flesh-being in space is expensive. I know it's easier to colonize Antarctica and the Gobi desert, or the Mariana trench for that matter. You may safely assume that's all occurred, or that Earth is a pristine nature preserve or whatever. Remember, ever spreading plague of happy humans.
And yes, I know the AIs and Uploads will probably take over, and that most intelligence in the universe in the future will likely be of the computronium rather then squishy fleshy brains variety, which would result in totally different mass requirements, I've read Age of Em too -- if it makes you happier, assume this is a pet-project of some trivial nostalgic fraction of the Uberminds. Let's just focus on the question at hand for now.