You may need to be a little more specific, however, I will give you one answer:
In regards to the U.S. National Firearms Act, there is no such thing as an NFA permit. In the U.S. in most states, any individual that can legally own a handgun can also legally own an NFA item, which includes short shotguns, short rifles, machine guns, supressors/silencers, and a few other items.
However, there is no "permit" for it. You must buy a $200 tax stamp and pass a background check, but it is literally just a tax stamp and the item is registered to you. You must buy a separate stamp for each item that you purchase.
Get an NFA Class II permit for one.
You must obtain an NFA permit before you do this.
You get and NFA Class III permit and buy a suppressor.
Only if you get an NFA Class II permit for it.
Yes, once you've obtained an NFA permit for it.
You can get them as short as 18 inches, provided you either mount them to a pistol lower, or else get an NFA permit to assemble a short-barreled rifle. For a Title 1 rifle not requiring an NFA permit, it's 16 inches permanent length.
Without requiring an NFA permit, the largest bore weapon a civilian can own is .50 calibre/12.7 millimetre in the US. Anything with a larger bore requires an NFA Destructive Device permit.
Any FFL dealer who possesses the Class III Special Occupational Tax stamp in order to be able to deal in NFA regulated items. They'll guide you through the NFA permit process. This is, of course, dependent on you living in a state which will approve NFA permits.
16 inches without requiring an NFA permit, the same as it is for the rest of the country.
16 inches before an NFA permit is required, just like the rest of the United States.
16 inches, just like in all the states. Anything shorter requires an NFA permit.
Yes, Pennsylvania permit all NFA items, except for explosive Destructive Devices.