The size adjustments are clearly marked on the cover over the analog board once you remove the back of the computer. You don't need to use an insulated screwdriver, but for the horizontal width you will want to use a plastic tool to adjust the ferrite slug. If you use a metal tool it will not only interfere with the adjustment, but it will get hot as the coil acts like a crude induction heater.

You'd just about have to *try* in order to get a 10kV shock, the EHT is pretty well insulated. The "low" voltage B+ on the monitor board typically packs a much bigger punch, but if you stay on the covered side of the analog board you'll be pretty safe. Just don't touch any exposed connections you see.