First, just to note that PTAREA is not one of the FITS header requirements for BeSS, in case this is why you are asking.
To calculate the area you use the formula for the area of a circle.
Area = pi x radius squared
The Celestron website gives the diameter of your C14 as 355.6mm, so the radius is 177.8mm. Thus the area would be:
3.14159 x 177.82 = 99315mm2
This does not take into account is the central obstruction in your C14. Again the Celestron website gives this as 114mm, so a radius of 57mm. Thus the area of the central obstruction would be:
3.14159 x 572 = 10207mm2
So the actual light collecting area of your C14 will be:
99315 – 10207 = 89108mm2