Manufacturers will provide recommended fixing positions to achieve optimum heat output from each radiator, this determines the height the wall brackets will need to be placed.
Wall brackets are usually sited a minimum of 300mm from the floor up to a maximum of 700mm.
Radiators heat the room most effectively when they are placed in the coldest part. Usually this is on an outside wall immediately beneath a window, where cold air drops to the floor.
You can, however, place a radiator on any wall if it does not suit your furniture arrangement or curtains to place them beneath a window.
If you do not have enough space for a single large radiator, consider instead hanging two smaller radiators of the same combined output.