I like doing quick calculations (it's how I figure these numbers out for my own games), so here's how I went about it:
-The population of New York has a density of about 27,000/sq. mile. Given 10ft ceilings (Tobias Caine is extravagant like that, less scrupulous RTs could fit a lot more soldiers into each barracks), that means that you could fit about 14,256,000 New Yorkers into a cubic mile (1 mi^3).
A lunar class cruiser is Space 75, and you can look up the rest of the space numbers for yourself (this is mostly to try and keep kosher with posting rules here, no idea what the site actually limits...but I'm just trying to be safe), and is listed as 5km x 0.8km abeam (no height, but lets average it out at 1km, since it looks taller than it is wide in the picture). That means the Cruiser has about 4km^3 space (about 1mi^3 according to Google). So 1 space on a cruiser = ~0.013mi^3
Given just those numbers, the average barracks on a Lunar-class Cruiser could be used to house about 740,000 New Yorkers (the population density already factors in municipality facilities, parks, shops, warehouses, ect).
So, by limiting myself to only ~100,000 soldiers, I've left plenty of room for water reclamation, large training halls, parade yards, vehicle repair facilities, armories, transportation, and other items necessary to running an active army-sized military detachment.
Personally, I think its great, since the logistics of trying to get a force that large off the ship and onto a planet would already require some thought, but it provides a strong backbone to work with. Now, it doesn't quite meet the books, where warmasters are sometimes shown invading planets with only a couple regiments, but given the wars fought throughout history, that doesn't bother me all that much. Plus it makes taking on enemy ships with a full barracks onboard much more scary.
It also means that you still need a fleet of twenty or so dedicated barracks ships just to move an invasion fleet around (4 barracks per ship, 10mil. soldiers).