From the 1965 Plymouth service manual, the default Fury tire size was 7.35-14. Reading between the lines, I presume that was for slant-6 cars. Station wagon (apparently slant-6) was 8.25-14. V8 cars (not wagons) was 7.75-14. V8 wagons was 8.55-14. This was for all Fury's.
I don't have a '66 Plymouth manual.
The '67 manual shows that the Fury's would have had 14 x 5j rims, which I believe are 5" (not 5.5" ?) wide, but wagons would have 14 x 6jk (all models would have 15" rims if disk brakes).
Again from the '67 manual, Fury's with slant-6 or 318 had 7.75-14 tires, if 383/440 then they had 8.25-14. This is all assuming drum-brakes. Trailer-towing package also gave you 8.25-14.
There is a chart in the transmission section that shows the tire size that corresponds with with the number of speedo pinion gear teeth and gear color. You'd have to take your gear out and count the teeth, note the color, and also know your rear-end ratio in order to see what tire-size that combo was set for.
Do you have the original rims on your car? Are they 14 inch? Do you know the width? They would be either 5 or 5.5 inches. If 5 inches then your car was set up with 7.75-14 tires, if 5.5 then it would be 8.25-14.
As Cbody67 says, closest metric size to 8.25-14 is 215-75-14. I'd have to do some digging, but I'm guessing 7.75-14 would be 205-75-14.