You gotta consider sidewall. The larger the rim, the thinner the sidewall, which means less movement, better handling, but more NVH. Also, the larger the rim, the heavier.
15" is best for rally and dirt roads.
16" and 17" is the best over all for most road use.
18" is best for track only and unfortuanly, very expensive.
17" is usually the prefered by most.
Width wise not too sure, depending what you want. Of course the larger width the more traction, but increase in NVH. Plus more $$!!
But don't skim on the tires, after all, they're the only physical thing between your car and the ground.
To credit my source: Training WRX by Nick Warne
"We live for a good time, not a long time."
RIP Colin McRae