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Is this something that will be repeated a lot? Are you using the GUI and an edit session?
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.
Are you thinking of a GUI tool/interface?