Saturn's equatorial radius is 60,268km, and its polar radius is 54,364km -- nearly 10% smaller.
The equatorial radius would have been smaller without the rapid rotation, and the polar radius larger, so at least we have lower and upper limits as to the size of a non-rotating Saturn.
Getting a more accurate estimate would require calculating the effect of gravity vs. centrifugal force on the density of the various gaseous layers. As long as you limit the calculation to the current state of Saturn (as if it had just stopped) and not try to extrapolate over its entire age of 4.6B years, the result is unlikely to be more than a couple of percentage points different from the current average radius.
The equatorial radius would have been smaller without the rapid rotation, and the polar radius larger, so at least we have lower and upper limits as to the size of a non-rotating Saturn.
Getting a more accurate estimate would require calculating the effect of gravity vs. centrifugal force on the density of the various gaseous layers. As long as you limit the calculation to the current state of Saturn (as if it had just stopped) and not try to extrapolate over its entire age of 4.6B years, the result is unlikely to be more than a couple of percentage points different from the current average radius.