Coil pack

I bought a set of new spark plug wires 8.5 mm and my mechanic put them on. Than I started up my car and it wont even start. So my mechanic put my old spark plug wires 7 mm and the car started right up. Are the new wires to powerful for the coil pack and if so what other kind of coil pack can I buy? My mechanic told me to buy the 8.5 cause they will last longer and they are thicker coated.