Getting radius around multiple transform

I do not know how your code is set up, so this may not fit exactly, but hopefully it will get you moving in the right direction.

 // The number of hexagons on the screen.
 int numHexOnScreen = 0;
 
 // The position of the cicle base on hexagon tranforms.
 Vector2 circlePos =  new Vector2();
 
 // The radius of the cicle.
 float radius = 0;
 
 void Update()
 {
     // First, add a hexagon and increment numHexOnScreen.
     AddHexagon();
 
     // Then, calculate the new position of the cicle.
     FindCirclePos();
     
     // Finally, increase the radius if needed.
     FindRadius();
 }
 
 void FindRadius()
 {
     // The distance between the circle's position and the position of the newest hexagon.
     float proposedRadius = Vector2.Distance(circlePos, hex[numHexOnScreen - 1]);
     
     // If the proposed radius is greater than the current radius, expand the current radius.
     if (radius < proposedRadius)
         radius = proposedRadius;
 }