The part I put in bold is what explains the best gear choice for skid(z).

Basically, the amount the road "pushes back" (the friction of your tire on the road) is always going to be the same. Therefore, the amount you need to RESIST the road's friction to lock up your back wheel will also stay the same all the time.

The difference is how hard you need to push ON THE PEDALS to put enough force on your back wheel to break traction. Like someone else said above, this is related to how hard you need to push FORWARD on the pedals to get moving. If it's really hard for you to pedal forward from a stop, it's going to be really hard to pedal "backward" (i.e., stop the pedals once they're spinning) to do a skid. The easier it is to pedal forward, the easier it will be to stop the pedals for a skid.

So, throwing some numbers into the mix, you either need a "smaller" (fewer teeth) crank gear or a "bigger" (more teeth) gear on the wheel.

Gearing terminology is always goofy. To make it simple, a gear ratio that gives you a higher top speed (like the gears that racers use in the velodrome) will make it harder to skid. A gear ratio that gives you a lower top speed (like the gearing that single-speed mountainbikers use to allow them to climb steep hills) will make skidding easier.