Assumption: Piece A is cut as a 5.5 inch square and "finishes" at 5 inches.
To enlarge the throw size from approximately 51 x 58 to 60 x 68,
I would add one "row" to the length and cut each of the borders one inch wider (7 + 1 + 1)
I would add two "columns" to the width and cut each of the borders one inch wider (3.5 + 3.5 + 1 + 1)
This would end up being approximately 60 x 67.