Kicad trace thickness and autorouter
In the PCB-New manual see NetClass. You open the Design Rules menu and choose Design Rules to open the Design Rules Editor dialog. In the list at the top there is one defined NetClass called default. Create a new NetClass called power with the track width etc you need.

Below this is a pair of lists surmounted by combo boxes both defaulted to "* (Any)". Set the right hand one to your new Power NetClass and then move the power pins to this NetClass by selecting them and clicking the >>> button.