You can´t fit bigger wheel then 17" if you have the standard suspension.
But if you get coilovers, the you can out 18" wheels on, but not wider then 7,5" and in the right offset. You may have to roll the arches a little bit.
"Understeer" is when you hit the fence with the front of the car.
"Oversteer" is when you hit the fence with the rear of the car.
"Horsepower" is how fast you hit the fence.
"Torque" is how far you take the fence with you.