The dims are as follows.

12-18x38 pieces for shelves.
2-18x72 pieces for sides.

the back is open on this rack if your gonna close it up then use a 39.5x72 back piece of pegboard. the pegboard will keep it form begin HEAVY and cheaper.

With mine open back with the 2x4s its still a heavy monster. Weighing over 200 lbs.
Heres a 3d solid model of mine.