From
the Disc Brake R&R document on the site:
I think twin piston is HD? Which would be 29.97mm/1.18".
That said, if it's deeply pitted, I'd be careful turning those... Especially if there's a lot of rust in the vents (which will cause breakthrough issues if turned too close- Had that happen on a couple of past vehicles). And the thinner the rotor, the more likely they are to warp.
1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).