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so, you already found out about contours & boundingRects, great !

now the only thing left is to find the largest one, which is not difficult at all:

double maxArea = 0; // keep track of the largest size
int maxId = -1;     // which one.
for (size_t i = 0; i < contours.size(); i++)
{
      double area = boundRect[i].width * boundRect[i].height;
      if (area > maxArea) 
      {
            maxArea = area;
            maxId = i;
      }
}
if (maxId != -1)
{
      // safe to assume, that contours[maxId] is the larest of them !
}

so, you already found out about contours & boundingRects, great !

now the only thing left is to find the largest one, which is not difficult at all:

double maxArea = 0; // keep track of the largest size
int maxId = -1;     // which one.
for (size_t i = 0; i < contours.size(); i++)
{
      double area = boundRect[i].width * boundRect[i].height;
      if (area > maxArea) 
      {
            maxArea = area;
            maxId = i;
      }
}
if (maxId != -1)
{
      // safe to assume, that contours[maxId] is the larest largest of them !
      rectangle(drawing, boundRect[maxId].tl(), boundRect[maxId].br(), Scalar(0,200,0), 2, 8, 0);
}

so, you already found out about contours & boundingRects, great !

now the only thing left is to find the largest one, which is not difficult at all:

double maxArea = 0; // keep track of the largest size
int maxId = -1;     // which one.
for (size_t i = 0; i < contours.size(); i++)
{
      double area = boundRect[i].width * boundRect[i].height;
      if (area > maxArea) 
      {
            maxArea = area;
            maxId = i;
      }
}
if (maxId != -1)
{
{ 
      // safe to assume, that contours[maxId] is the largest of them !
      rectangle(drawing, boundRect[maxId].tl(), boundRect[maxId].br(), Scalar(0,200,0), 2, 8, 0);
}
} // else assume, it did not find any contours at all. can't rule that out !