I found out that 5/8 works. Though Zetec Racing told me it's 16mm which is probably more correct. I only break the ceramic when I'm getting them out. Seriously, I broke the ceramic on one plug and messed up my spark plug socket, but I got it out.
"Wow, that sank fast." "Yeah, like a rock." 96 Contour GL ATX 9mm Ford Racing Wires; 2 Resonators down, 1 to go;"Special airbox"; many mods planned Sony CD player: 50wattsx4;remote; Speakers=50 watt Pioneer 6x8s Ross Evans