After the settler, start building a temple, and then library and marketplace. Meanwhile, build roads and irrigation on selected tiles to increase trade and food production. Once you have marketplaces in all your cities, you may consider switching to Republic and raise luxury rate to prevent revolt. By now, your cities should be at least size 3, and you can grow them by celebration if they have enough trade and food surplus. Next you want banks to boost luxuries even further, so that you can lower the luxury rate and get more cash to buy aqueducts to grow to bigger than size 8.
Stock exchanges serve a similar function.
Then you will need sewer systems in order to grow to over size 12. With a large population, the city improvements should pay for the themselves very quickly. In order to grow even further, farmland and supermarkets will be needed to produce more food. When all grasslands and plains are turned into farmland, cities can easily grow to size 30 and above. And from size comes power.