We make our lap quilts for the church 45 by 54. 11 squares across by 14 down would give you 44x56. With a moderate sashing, that'd make a decent size for most men. 11 x 14 = 154 squares. Did grandma use sashing? If not, I'd add another row both ways, so it'd be 12x15 which is 180 squares.
What a nice idea to make him what HE wants.