The stock tire size is printed on the label inside the driver's door jamb on both of my trucks.
I have the stock spare on my 98.5 and with the limited slip differential I plan to keep the same size on both sides on the back if I have to drive very far so if I have a flat on the back I would have to change two tires (move a front one to the back).
It is also a function of the tire manufacturer. My stock Goodyears (245s) stood 29.5 " tall (with the truck sitting on the ground. My Michelins (265s) stand 31" tall. That is more difference than applying the aspect ratio formula would indicate. My Superchips programmer asks the standing height, not the circumference. My speedometer and my GPS agree so; at least in my case they must have hit it right.
If you really need the static circumference measure the diameter (front to back through the center) and multiply by pi (3.14 is close enough) or run a rope around the unloaded tire and measure its length (around 100" for near stock sizes).
The dynamic circumference is a function of speed (centrifugal force).
The tire manufacturer's web site might give you the diameter, the circumference, or the revolutions per mile at a particular speed.
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