It's a great historical city to live & work where I've been staying for almost two years and still loving it =)
I can tell about Novi Sad (250k people, nice university, nice city center etc.), you can indeed get a livable flat for one or two people without kids for anywhere between 20k-35k euro. Though there are still some downsides to living in Serbia.
However, I was born and I still do live here as a freelance software developer (maybe at 30 hours/week, including time for learning new stuff), mostly Rails, nothing fancy, and I can tell that with a family of 4 I'm still saving 75% of my after tax income. Without trying hard to save money. That's what's keeping me from moving westward.
I'd say that medium town is from 50K to 200K people. Above that I'd call them cities but I don't think those are strict terms.
Here's a list of polish towns and cities: https://en.wikipedia.org/wiki/List_of_cities_and_towns_in_Po...