From a comment chain that got too long and should have been an answer (thanks @jamesqf for the conversation):
Using 21st century sustainable industrial farming practices, you will need 4 acres of land per person to keep people fed. If fresh surface water is used for irrigation, the farms should remain productive for a very long time.
Since modern industrial farming is built on recovery from the bad practices of the 19th century that led to the dustbowl years and widespread famine, it follows that adherence to the practices improves the soils year after year within the parameters of the indicators that the farmer knows to check and manage.
The caveat of "what the farmer knows to check and manage" is crucial. For example, one of the challenges faced by farms that are dependent on well water for irrigation is the trace amounts of salt in the well water build up in the soil over time and poison the land. 21st century agriculture is struggling now with the question of how to economically desalinate soil at scale.
If pre-1940 practices are followed, you can expect to require 15 acres of land per person, with the caveat that the soils will be depleted within 150 years +/-, and the city will be forced to either acquire more land or develop farming practices analogous to 20th century industrial farming.
This allows no buffer for contingencies (disease, pests, accidents).
My town is home to about 50,000 people, and covers 31.67 square miles. It is not incredibly densely populated, but if it were more densely populated I would not want to live here.
So, using 21st century level industrial farming, a marginally self sustaining city of 60,000 people will require 375 square miles (a square 19.4 miles to a side) of farmland plus 32 square miles of living and working space. 407 square miles
Using 19th century farming practices, a marginally self sustaining city of 60,000 people will require 1406 square miles of farmland plus 32 square miles of living and working space. 1438 square miles
Keep in mind that this is marginally sustainable - a bad crop year would wipe out your city, so you would want to double this and have other plans in place for contingency.