The height of an upright is determined by quote a few factors. These are:

Height of the building to underside of sprinkler heads.
Type of sprinkler system in the building.
The height of you forklift being used to load the rack.
Most importantly, the height of the load including the pallet.
Please note these two variable are part of the fire code which states that the top pf the load on your pallet must be a certain distance from the underside of the sprinkler heads. Thais can vary anywhere from 18″ to 36″ and sometimes as much as 60″.

Once you have properly selected the height and depth of your upright you need to also choose the proper gauge/capacity of that frame, as well as the proper size foot plate.