I always had the impresion that space stations, the ones we use to dock ships in the game, have in the ballpark of 1million inhabitants.
That might seem a lot, but to me, it represents 35% of the actual people at any one time, since there should be quite a bit more of activity from transient population coming and going through.
Moons could have small settlements in the ballpark of 100k , just think in terms of mining towns.
Deadspace locations is harder to estimate, can't really say anything there.
And the big mistery, planets, Seyllin was like the huge mining heaven nearby the core worlds of the Federation, a mere 5 jumps from Luminaire, if it had 200mill it was because of 2 reasons, incredible rich and extended deposits of any valuable minerals, and "walking distance" to a more decent place. That meant people could do shift of 6-8 months and come back home for 1-2 as R&R.
I would not stretch said figure and apply to most worlds out there. Temperate planets sure would seem to provide the framework to sustain large populations, but remember that what can be considered temperate worlds could be planets in the process of terraforming, with their atmosphere partially completed.
Also you need to consider the following, Gallentean/Caldari diasporas were frenetic, they acquired half of what the Amarrian manged to accomplish in less than a quarter of time. So i bet that both core worlds were already saturated and overpopulated. Probably birth control already taking place.
A well established planet could probably house 600-900million, less developed, 200-300million, core worlds 8-9billion.
Just applying a bit of ideas and estimate though, nothing is backed by PF.