I've found the 12" clutches are usually on the heavy-duty rigs, 1 ton and above. There is an 11" option that's available, and also a 10". Your best bet is to take it out and measure before ordering parts. Be sure and remove the flywheel and get it resurfaced, and replace the pilot bushing in the end of the crankshaft. The clutch plate can be installed backwards, and the center hub will drag the flywheel attaching bolts and prevent the disc from releasing fully.

Use an alignment shaft during reassembly to align the clutch and pilot bearing, and it's also effective to use a couple of long, headless bolts screwed into the bellhousing to guide the transmission back into place during reinstallation. Tighten the pressure plate bolts a little at a time to avoid distorting the clutch cover or misaligning the clutch plate on the flywheel. Good luck!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!