ST_MinimumBoundingRadius — Returns the center point and radius of the smallest circle that contains a geometry.
(geometry, double precision) ST_MinimumBoundingRadius(geometry geom);
Computes the center point and radius of the smallest circle that contains a geometry. Returns a record with fields:
 center - center point of the circle 
 radius - radius of the circle 
Use with ST_Collect to get the minimum bounding circle of a set of geometries.
To compute two points lying on the mimimum circle (the "maximum diameter") use ST_LongestLine.
Availability - 2.3.0