These were custom built wall to wall so my dimensions will be different from yours, but to give you an idea. I went with 16.75” wide for each locker (the length of the wall – 2 inches wiggle room – thickness of material / # of lockers). x 22.5” deep (as deep as you could comfortably reach into the locker) x 71” tall (yup, a grown man can fit in our lockers, here’s proof).
I opted to use a different material than I had originally planned because I found these 3/4” birch / melamine on clearance at my local lumber yard, MacBeath’s. Finding a deal on this sheeted plywood saved me $300 on this project, but also altered my building plans to make the project a little more complicated. You see, with plywood that is different on each side it had to construct each locker individually sandwiching the melamine between the lockers. If I would have done this project using 2 sided plywood (same wood on both sides) I could have made each locker about 1 1/2” wider, and all of the lockers combined! See Example below of the differences in construction of the mudroom locker.