7mm's are alive and well, got a set on my stang and they seem to work fine, bought them because went to DS2 and have not got the oil leaks under control yet so I didnt want to ruin an expensive set.
For the most part the core (conductor part) of the wires does not change based on the 'size', a 10mm wire could have the same size core as a 7mm. This is not always true but for the most part seems to be. The main reason you get a thicker wire is to try and keep the spark inside the wire and to the plug instead of jumping to something else nearby like another wire or some other part under the hood. The higher the voltage the farther it can jump. Things like moisture make it easier to jump, thats why if you live in a wet climate you MAY have some benifit to a thicker wire. The core of a wire is a fairly brittle and sensitive to heat and other contaminants, a thicker wire would therefore offer more protection to the core. That being said the larger wires tend to be better quality as far as ends boots and other materials and workmanship.
As to coil voltages.... MOST (but not all) of the stuff that you would buy in a average parts store (from stock to a blaster/flamethrower type) is not going to have enough of a differnce in voltage to really stress things like wires in a typical enviorment/ application. There are big differnces in coils but the voltage is one of the less critical issues for most stockish street applications.
You do forsure want to gap the plugs to whatever the petronix says. The coil has less to do with the gap, its mostly up to whatever is doing the control, points, DS2box, hei module, or in your case petronix box/module.
Now if you are running 3 turbos at 8000 rpm you need to do some more research but if you got a stockish engine on the street you will be fine with just about anything you can buy these days.
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