There are a number of factors that must be considered when establishing the size of rack uprights and beams. The upright depth is driven by the pallet size and the insertion depth. Rack is rated with the presumption that the pallet stringer or box supports rest on the beam. The typical design works with a three inch overhang of the pallet front and back. This provides a safety margin for the driver when placing the pallet. Having the face of the pallet flush with the beam creates a risk that on pick up, the driver may accidentally push the pallet in back of the beam and allow the backrest inside the rack opening. If the pallet is lifted in this position, there is a danger that the backrest will contact the beam above and dislodge it leading to a collapse of that beam and a falling pallet.
When multiple pallet sizes are anticipated, it may be appropriate to use more than one upright size if decking and/or pallet support bars can not assure that the pallet weight is properly transferred to the beams.