Width
Measure beyond pipes and valves on both ends. Choose width that gives you enough room for fit and balance.
Height
Measure from floor to top of radiator. Add one inch and choose nearest standard height to fit.
Depth
Leave ample room both in front and in back of radiator - choose depth which is larger than radiator.
Never order a radiator enclosure that is exactly the same size as your radiator, or a smaller size. Make sure the size you order is at least 1 inch longer, 1 inch higher, and 1 inch deeper than your radiator.