Ignition Leads
 

I wanna replace my leads, atm I have Ultra Tune leads on my rx7, 8mm.

I don't know to much about the leads so can anyone tell me if I can go bigger or something? I got a set of new NGK'r sparks so I've put them in but I want new leads as well. How big can I go? does it matter? I was looking at a set of Bosch leads or something. 10mm?