The diameter of the tire doesn't change no matter what the size of the hub, it is still a 35 inch diameter tire measured from side to side across the middle, the hub simply takes up the center section and fills the "hole" in the tire.
Looking at the material in the WOF link Chris provided, the standard Electra tire was a Goodyear "Airwheel" - "Goodyear Airwheels and heavy duty tires were standard equipment on all Lockheed Model 10 Electras. The size was 35x15-6. The 35 refers to the uncompressed height of the tire, the 15 refers to the width across the face of the tire, and the 6 is the diameter of the opening in the middle of the tire, all in inches. Lockheed publication EE1135 revised 5/1/36."
So if the hub is 6 inches across, you'd have 14.5 inches of rubber sidewall on either side of the hub for a total of 35 inches in diameter.
Andrew