City progression population requirements (current level - population required for next level - next level):
lvl1 - 50 - lvl2
lvl2 - 150 - lvl3
lvl3 - 300 - lvl4
lvl4 - 600 - lvl5
Thus, if you have a 3-grain city and you want it to achieve its fourth size category, you need to have a total food per grain of 100. This can come from other towns by means of the Grocer line of buildings (+5/10/20/40 faction-wide food per grain per structure, affecting both the town it's built in and all other settlements in your empire; additionally, the +40 faction food per grain is a 1-per-faction structure), the Garden line of buildings (+15/35/50/60 local food per grain; additionally, the +60 food per grain is only available to Empires), the Well line of buildings (+25%/50%/50%/50% local food production, so there is no population bonus to upgrading beyond the Inn), the Pier line of buildings (+5/5/10/15 local food per grain; additionally, the final upgrade of the Pier is only available to Kingdoms), the Great Mill wonder (+25% local food production), and a couple of spells (+25% food production per essence from Gentle Rain, +1 grain from Nature's Bounty). Possibly also a few other buildings I can't think of right now, and it should be possible to create unit abilities via modding that would add to faction food production, as well. The Well and Grocer lines of buildings can only be built in Towns, and Well line buildings do not provide increasing food bonuses after the first upgrade.
Let's assume that we have an empire of 20 cities, which includes ten Towns, seven Conclaves, and three Fortresses. Five of those towns have Grocers, which together grant +25 faction food per grain (+5 food per grain * 5 grocers). Another two of the Towns have Bakers, which together grant +20 faction food per grain. One Town has a Butcher, for +20 faction food per grain, and one Town has a Brewery for +40 faction food per grain. The other town doesn't have any of this stuff. Thus, from our ten Towns, we have a bonus of 105 food per grain in every settlement our faction has. Neither Conclaves nor Fortresses are capable of affecting the food production of other settlements, so we'll ignore them. This faction-wide food bonus from the Towns is sufficient in and of itself to allow any tile with three or more grain to support a level four city, any tile with two or more grain to support a level three city, and any tile with one or more grain to support a level two city, without including local food bonuses from Garden-line or Pier-line buildings or the city hub itself (which provides 25 local food per grain).
For the most part, though, I agree with Fallenchar that a 'practical' example isn't really useful. The age of your empire and how you've been building it, and the type of city you have, and the amount and focus of your research, are all going to impact what you can do at any given time. I've had games where I could basically ignore food-production centers because I'd built enough Towns with Grocer-line buildings that even my 1-grain cities could reach level five (eventually), and I've had games where if I wanted my cities to grow a size level I'd need more food structures than would reasonably be available to my faction in a worthwhile timeframe.
Also, growth bonuses from food surpluses are essentially the food surplus percentage divided by 100, restricted to be between 1 and 3.