I've created a city at the joining point of two rivers, and I'm trying to determine the population ceiling. I'm just going for a general idea of relative size, and so far I've just been making towns and settlements get larger as I go downstream from the mountains to the ocean. This city I justified the existence of with the settlements established on its adjacent river's tributaries. There will be another, even larger city near where the river meets the ocean. However, I've begun thinking that it may be more logical to have this first city be the largest because it is central. This question is essentially: is upstream trade in my world viable enough to cause the more central location be more populated rather than the downstream location?
My world is a fairly standard dnd 5e feudal society, with a lot of leniency to make room for all the homebrew bits of magic I add in. There are no magical vehicles or beasts of burden that could be used as a fantasy equivalent to steam power.
Edit: I basically didn't realize that you could tow barges with animals and I thought that upstream trade was a lot harder than it actually is. Thanks so much for the answers guys, I'll have to start doing some more research into the economics of medieval cities.