Well, the equation is great. But you need to know its radius. Fortunately, good old google has a decent approximation for the radius of the sun...
So, if you copied your formula down right, and:
A = pi*r^2
Then:
A = (pi) * 695,500km^2
I'll let you use your calculator to figure that out.
As far as diameter... well, just multiply the radius (which I already looked up for you) by 2.