KiCad works with nano meter resolution. And that is quite small.
You are a relatively new member on this forum, so I’ll assume it’s a basic question.
Learn to work with net classes. A net class is a set of rules for track widht and clearance and via size. You can make new net classes in: Pcbnew / File / Board Setup / Design Rules / Net Classes. Click on the plus sign on the left just below the main “table” area to add a new net class, then adjust it’s parameters.
The lower half of the dialog is for managing your nets into the net classes. On a new PCB all nets start with the “Default” net class.
Also be aware of where you have your PCB manufacturered. 0.2mm is still a value most PCB manufacturers can handle relatively easily, but if you go much narrower you get into an area where you need more specialized services (which are likely more expensive). Somewhere around 0.15mm your options start shrinking.
The “Minimum track width” (which defaults to 0.2mm) is set up on the main page of the the design rules: Pcbnew / File / Board Setup / Design Rules