A 15" block drawn on 4 squares to the inch graph paper would be 15 x 4 = 60 squares.
On 5 squares to the inch graph paper, 60 squares is 12"!
If you draw each piece on 4 squares paper, then count the squares to redraw on 5 squares paper using the same number of squares between lines, you will have your 12" block pieces.
You WILL have to redraw the 1/4" seam allowances.