Tailspin Turtle
I thought the J91 had a diameter of 55 inches, and the J58 was 80% the size of it.
OldBUFFkeeper
I found additional data for the JTF-17 turbofan
Overall Length: 215"
Inlet Diameter: 69.3"
Reverser/Suppressor Diameter (widest section): 80"
Compressor Configuration
LP: 2-Stage Fan
HP: 6-stage compressor
Combustor Type: Annular
Turbine Configuration
HP: 1-stage
LP: 2-stage
Mass Flow: 687 lbs/sec
Bypass-Ratio: 1.3:1
Pressure Ratio: 13:1