WWTVICTOR
Webroamer
I actually did a quick mod for this where I had a Sumerian city with a granary, hospital, the works, smack-dab on top of a wheat-filled floodplain, surrounded by wheat-filled floodplains, with railroads and irrigation on all of them. I think I got up to...maybe 63 or 64 citizens? That's the highest theoretically possible, at least...but of course it's cheating, and I don't know what the limit is in terms of what the game would ever give you.
Just mod it to let the city have floodplain and river tiles (all identical), and wheat on each tile, set your civilization to have Agricultural, Industrious and Commercial, irrigate them as fast as you can, and build all improvements like granny, Shakespeare and railroads asap, and don't forget all commericial wonders. Last but not least, create a government type that can upkeep all units, minimal corruption, commercial bonus, etc., just benifits you is okay...Let's see how many food and commerce you will produce!
Note: If you don't want to practise, just tick all the wonders and improvements when you are modding and do all the procedures in the mod (although my procedures are just alike with yours).