I didn't have this problem in the past, but some time over the last upgrade or two, or perhaps due to some option I selected/deselected accidentally, I cannot find a setting in the Brush Panel in Photoshop that turns off brush size reacting to pen pressure. I only want opacity to respond to brush pressure, but Shape Dynamics always opts to vary brush size, as well as opacity, with pen pressure.
Any suggestions as to how I should set the brush preferences to avoid size changes?