OSH Park setup for KiCad
I am not sure this is a good question for EE.SE, but here's how to do this in KiCad:


From the Design Rules menu select Design Rules
Choose the Global Design Rules. There you can modify available trace widths/vias.


The Custom Track Widths lists the available track widths (i.e. outside of those defined for specific net classes [see below]. And the custom via sizes lists lists all the vias you want to have available while routing.  Thus, in the following dialog box trace widths of 10 mils, 15 mils, 20 mils, and 32 mils are made available for selection while routing, but only one via size.



You can also 


Set the vias/trace widths for specific net classes in the Net Classes Editor tab of the (Design Rules) dialog.  This is a PITA, however, since KiCad makes it a bit difficult to assign sensible names to net classes (as far as I know). You can, however, edit all the files by hand (all KiCad files are text files), so with some regular expressions, this might be a way to go.


After doing the initial setup, the second toolbar, on the left, will offer choices of trace widths and vias you can use for routing, as in the following picture: