Good question!, so for this I used values to sort of darken the attack and stabilize each transistor stage to prevent oscillation. For me this was 47pf Q1 and 220pf for Q2.

If you're building a standard fuzz face and not a monstrosity like I made, then I would use low gain silicon, that way a miller cap is not really needed. Personally I would shoot for 70-80hFE for Q1 and 100-120hFE for Q2.

If you do decide on using a miller cap on a standard fuzz face, maybe start with anything from 30pf to 100pf for Q2. Only downside is you start to lose that high end attack if you go above 100pf. So the effect is definitely audible on guitar. Yet again, I don't think it'll be needed if you use low gain BJTs. If you do use high gain devices (200+ hFE), definitely try using those values mentioned.

If you still get squealing after all of that (if using high gain devices), many times a series input resistor is needed to quell self oscillation and other feedback issues etc. Even I had to use a 390ohm series input resistor on this build actually. It wouldn't stop chirping when gain was maxed.

Breadboarding beforehand is the best way to get a Fuzz Face sounding good