A city built entirely in irrigated and railroaded floodplain can grow to size 51, plus one for each wheat. You're highly unlikely to see an all-floodplain city location in an unmodified map, though. If modifying the map and not the base rules is allowed, the biggest city possible is a city of all floodplain with wheat, which could grow to size 71.
When not using a custom map, you'd be doing well to get a city past size 45. That would be from all irrigated and railroaded grassland with four cows/wheats.