Spring diameter is a big factor mainly on small block Chevs, Fords and Mopars. Many engine builders stick with a relatively low ratio rocker arm and make up the lift with the cam lobe. The rocker arm itself is short so that if a 1.50 thru 1.60 ratio is used there is pushrod clearance. Because this rocker arm is “short” there is a limitation on the spring diameter that can be used. For 1.450” fulcrum length rockers the maximum spring diameter is a 1.550”. If the geometry is correct, the rocker will have plenty of clearance to the spring and retainer. If a 1.450” long rocker is hitting a 1.550” diameter spring and retainer combo, the usual reasons are the stand is too low or a plus height keeper or retainer have been used to increase installed height.