If you know the area you want then you can work out the radius that you need to create it
Area of circle = PI * r^2
Therefore to know radius the equation is
r = SQRT(area/PI) where SQRT means Square Root.
So for example if you want a circle that is exactly 200m2 then the radius is SQRT(200/PI) which is 7.978m to 3 decimal places.
Now you know the radius just feed that into a buffer tool.