I found this on farmland necessary to sustain some amount of people: https://worldbuilding.stackexchange.com/questions/9582/how-many-people-can-you-feed-per-square-kilometer-of-farmland so using conventional farming (no hydroponics, magic, or crazy stuff), you'd get around 2350 people per km<sup>2</sup>. Flip it around and that's about 425 km<sup>2</sup> for a million people.

Ok, then I looked up population density for one of the major US cities that I'm familiar with. About 3662 people / mi<sup>2</sup> or 1400ish people / km<sup>2</sup>. That's about 715km<sup>2</sup> for a million! This metric isn't ideal at all since medieval london was around 100 people/km<sup>2</sup> which is a huge difference so you'll have to play around with this number. I read that ancient Rome (city of Rome) had around 1 million inhabitants so that might be a good reference as well

So, 425 + 715 = 1140km<sup>2</sup> which is a circle with a diameter of 38km or 24mi. That's probably a solid start to the problem. You'd probably have to also look up how many farmers you'd need per km<sup>2</sup> to help estimate that layer.

oh and the average human walking speed is about 3.1 mph so on a clear cross-cutting road, it'd take 8 hours to make it across with no breaks or obstacles.