Hello!
How are you obtaining the centroid of the object? Do you use moments? Do you have any additional information like the variance of the object pixels, size of the blob, it's principal axis length...since any of them can be used to find the radius of the circle you intend to draw. Once that is known, we can draw a circle with the centroid as center and the radius on each frame as desired by you.
Kindly let me know if it helps. Thanks!