The volume will decrease.
If everything else remains the same then it will decrease. If other sides were increased proportionally then it would stay the same. It would increase if other sides were enlarged more than the length was shortened.
Width of 76 inches and length of 3
or width of 77 and length of 2
or width of 78 and length of 1
they all have a perminiter of 158 and all widths exceed 75