x * size <= width                  //x - number of columns
y * size <= height                 //y - number of rows
x * y <= N                         //N - number of squares
size -> max                        //size - size of squares

1 1 1 1
1 1 1 1
1 1 0 0

// to make things more simple I put width as bigger size
int biggerSize = this.ClientSize.Width;
int lowerSize = this.ClientSize.Height;
int maxSize = int.MinValue;
int index = 0;
int index2 = 0;

// find max suitable size
for (int i = _rects.Count; i > 0; i--) {
  int size = biggerSize / i;
  int j = (int)Math.Floor((double)lowerSize / size);

  if (i * j >= _boards.Count && size > maxSize) {
    maxSize = size;
    index = (int)i;
    index2 = (int)j;
  }
}

int counter = 0;

// place all rectangles
for (int i = 0; i < index; i++) {
  for (int j = 0; j < index2; j++) {
    if (counter < _rects.Count) {
      _rects[counter].Size = new Size(maxSize, maxSize);
      _rects[counter].Location = new Point(i * maxSize, j * maxSize);
    }

    counter++;
  }
}