What size tires are best for dirt traction?
Wider tires like a 205/60 r13, at low PSI, or skinnier tires at normal PSI? Or something else?

 

Depends on what tires you're running.

If you're using rally tires, normal width is good, since they have a tread designed to grab dirt. If you're using snow tires, you will need a little skinnier. If you're using all-seasons, your driving style is a lot more important than the exact width of the tire, so whatever you got.

As for tire pressures, use whatever pressure they recommend. Snow tires, usually the same deal, as most snow tires have recommended pressures that are lower than normal. All-seasons, lower than normal, by 5-10 PSI, depending on how close you normally run them to max. Make sure you don't have the pressure too low, or you'll knock the tire off the rim.