A Full city is a city with apoximetly 20,00 Men in troops. That is NOT the same as 20k troops, as all non-Meele units cost more than one Unit of Population. Although the Level 30 Upgrade of the Farm holds 23,000 appoximetly 3,000 are used in building your town. Some people chose to Have less Buildings upgraded or fully upgraded due to the ability to get the additional men in troops. I beleive that units that are sent as support are from a differnt city, and thus can go past that limit (not tested, can someone comfirm?).