No, this is not how to increase the size of a fantasy economy if you are a fantasy ruler. I could teach classes on that, but I won’t. This is how to take a couple of small things and start building them into much bigger things.
You need to start somewhere. Over 30 years ago, I drew a map of Fletnern, placed the major cities (poorly), and wrote really short descriptions of them. One of the items in those descriptions was what their major exports were. That’s a good enough place to start! Do you know what the natural resources or at least major exports of one of your cities or regions is? Good!
Let’s begin with an example. Two of the major exports from Rhum are beer and ceramics. We’ll start with ceramics. The soil in many places around Rhum is great for various types of ceramics. So, at some point over the last 30 years, I decided that inside the city of Rhum are several ceramics factories. They make different things, but one makes plates, another makes steins, and I forget what the rest might have been. So, we know that there are wagons filled with ceramics products moving out of the city. But in order to run a ceramics factory in the city, you need to bring in wagon loads of clay. OK - sounds a little weird, but we can do that. So now that you have the raw materials, there must be some pretty large kilns as well for firing those ceramics, so we’ll need some fuel. Rhum is surrounded by forests and the coal deposits are further south, so we’ll have them bringing in wagonloads of charcoal to fuel the kilns. The point? We started with “ceramics”, but now we know that plates, platters and steins are being crafted in the city. Honestly, they must be reasonably fancy or no one would bother “importing” clay - They’d just do it right there at the clay pit. So we know better what’s going out and what’s coming in - charcoal and clay. We’ve started.
But there should probably be that industry right there at the clay pits too. There, they make bricks. So we now also know that there are wagonloads of bricks floating around the city too. Moving on to beer: there are major breweries in the city. So the raw materials going in include barley, but do they? If you know anything about beer, they brew it not from barley, but from malted barley. So let’s have the malting process outside the city at the barley farms and plantations. So the thing coming into the city is the malt. What else do you need? Well the hops (they use hops in Rhum), is really minor, but there would need to be some of that. Also - there would need to be barrels. We’ll have the staves cut and dried in the field and then imported, so the barrel making is going in in the city. What about the hoops? saplings or metal? How about both?
OK - Without letting this get way too long what have we started to do? We started with ceramics and beer as exports, but now what do we know. We know wagons filled with clay, charcoal, wooden staves, saplings, copper, malted barley and hops are being brought into the city. We know wagons filled with fancy plates, platters and steins are going out as well as wagonloads of barrels of beer. But we know a lot more too. We know that the countryside is going to be filled with barley farms, clay pits, brick makers (with their own kilns), and colliers (that’s a charcoal maker). We know those barley farms have special buildings for the malting of the barley. If you want to get fancy, you know that there are ice houses near small lakes because they need the ice to control the temperature of the beer in summer (as opposed to brewing ales). That’s actually a lot about the culture of the region from two tiny ideas.
What’s next? Well, you know they grow barley, but what do they eat? Are the people heating their homes with charcoal too? with wood? Are they using horses or oxen to pull the wagons? donkeys? mules? Where are those animals bred and sold? Where are the wagons built? locally or are they brought from somewhere else? Beer doesn’t seem like a major export, because it is typically cheap, so this must be pretty good beer. Is everyone buying it, or just certain other cultures? What is blatantly missing? Well, iron and steel seem to be noticeably missing, so those must be coming from somewhere. Glass is missing, but if the ceramics are so good, maybe they don’t care. Maybe nobody in Rhum uses wine bottles. Maybe they don’t drink wine.
This wasn’t a lot of work. We knew some of the common exports. We assumed that the artful craftsmen were in the city (preferred to live in the city), and the cruder craftsmen lived outside the city where it must be cheaper. We brought the raw materials and supplies in. We defined the outputs just a little bit better. We wound up describing a whole bunch of stuff that people moving around outside the city would run into. We even defined a major section of the culture within the city because there are factories where large numbers of people work on the same goods. That means people tend to work for a major boss, and not for themselves, at least these folks do. Does that affect their lifestyles? Probably. By the way - One of the other exports of Rhum is furs, especially beaver. Even assuming that the pelts are coming in tanned and stretched, this is whole huge aspect of trade that must be going on. What’s going through the gates of your cities?