OrCAD PCB Designer PSpice Automatic Route
The stock auto router is not that great as people have mentioned.  I've seen it done well by guys who put a lot of time into it but it's often more work than is worth it for one off designs. Basically you have to constrain the heck out of it.  So have you tried going into constraint manager and setting either the default trace width or the trace width for that net and then running auto router?

It's under Setup->Constraints.  Then choose the physical section on the right hand side and you'll get a table for all routes or you can select Net and assign constraints to individual nets.  Orcad PCB/Allegro is all about setting constraints on your nets.  Things like width, number of vias, impedance, etc.  The fancier the package you own the more advanced the constraints you can set.  The better you constrain your design the better the autorouter will perform.