Smaller city for one month

Nîmes, Aix-en-Provence and Avignon.

A good friend of mine and her wife bought an apartment in Pau, France, and absolutely love it down there. They go down every year from January through May, and really enjoy it a lot. Pau has a population of about 80,000, so it would fit your qualifications, and it does have good rail connections. One of these days I'm going down to visit her, as she raves about the scenery and about the people and how lovely the place is.

Chartres. It's famous for its pilgrim cathedral, but there are a lot of areas of Chartres where you'll rarely see a tourist. I discovered there are two other major churches in the town that i found to be as interesting as the pilgrim cathedral. The city center, which some tourists never see, is as charming as any town in Europe. Chartres is 1 hour, 15 minutes from Paris by train, so it's easy to get to the city. To the west, Bordeaux isn't far. The metro population of Chartres is about 90,000.

Another suggestion is Bayeux in the Normandy region, another charming town. That's in northern France, and you'll be there in December, so it'll be cool.

No one has yet mentioned Carcassonne, so I will. About 50,000 population, lovely area, decent train connections via nearby Toulouse. It's a very relaxed place if you stay away from the medieval city, but I'd still recommend a day in that touristy hubbub because it's just so much fun. Walking along the banks of the river and the Canal du Midi is wonderful for those like me who enjoy riding shank's mare.

We stayed 2 nights in Pau this spring. Granted, that is nothing like a month, but we enjoyed it. You are close to the Pyrenees, if the weather allows you to go there. And for me the best part was an amazing bakery; I would go there every morning! Might not meet your needs, but I'd say it's worth researching.

Be careful about the winds that blow in December. The Mistral blows down the Rhône Valley, so Avignon and Aix-en-Provence can be very disagreeable in December about every third day. Nîmes has less wind. Carcassonne can have strong Tramontana winds in December. You'll avoid the Mistral and Tramontana winds in Montpellier, Sète, and the Riviera. Menton on the Italian border has the best weather due to its micro-climate. I'd look there for a month in December.

Thank you all for the information. You have given me some good places to consider. Also, I had forgotten about the minstrel and this will really influence where to stay

We spent a month in Narbonne, but in September/October, and not in December. At the smaller size of your range, but excellent rail connections. Other towns in the area that we enjoyed visiting to consider are Perpignan, Albi, Carcassonne, Beziers. Montpellier would be just outside your population range.

There are probably better options for finding lodging for a month, but we set filters in Airbnb (1 month stay and max price range), to look at the options across multiple cities. We happened to find a very reasonable stay in Narbonne.

Rouen is one of the smallest cities that has a metro line. It's not far from Paris and is small enough with some big city amenities