You should not write about size but rather track width. Size is not clear - it can be, for example, understood as a total length of all net tracks, I think.
If the 50 mils VCC or GND track is going through my 0603 or 0805 capacitor pad I just place that pad on that 50 mils track (on its shore).
KiCad has no autorouter. If you use any than you should search there for information how to specify such limitations.
The width of copper tracks is usually defined in: PCB Editor / File / Board Setup / Design Rules / Net Classes.
New in KiCad V6 is that you can also define the net classes themselves from: Schematic Editor / File / Schematic Setup / Project / Net Classes. You can adjust line width and color for the net classes to be used in the schematic.
There is also: PCB Editor / File / Board Setup / Design Rules / Constraints / Minimum track width. This is mostly to ensure that you do not draw copper tracks that are too narrow for your PCB manufacturer to manufacture.
Usually it does not make sense to specify different minimums for nets. For example if you have a +5V power rails, you probably want to set the net class to something wide, do reduce voltage drop. However, if you have some pullup resistors to +5V, it does not make sense to mandate thick tracks to those resistors.
KiCad also has: PCB Editor / File / Board Setup / Design Rules / Custom Rules, and one of the constraints you can use is “track_width”. Learning how to use those rules does take some effort though. There is a whole chapter for these rules in the KiCad manual (On my PC it’s in: file:///usr/share/doc/kicad/help/en/pcbnew.html#_custom_design_rules) and there is also a “syntax help” in the upper right corner of the rules editor itself.
When FreeRouter was called Superrouter (many, many years ago), it was able to accept the PADS definitions that included the standard width and the minimum width. It used to try first with the standard but it had the allowance to reduce the width when required. Freerouter seems rather similar to the old one, but probably this feature has been dropped.