nParticles are created with Radius, Mass, Opacity, Color, and Incandescence per-particle ramps already added to the nParticleShape node. You can delete these per-particle attributes by setting the ramp's Scale Input value to Off (Constant for Color).
If you want to use an expression with these per-particle attributes, you need to manually add Radius, Opacity, Color, and Incandescence again. Like other per-particle and dynamic attributes, you can add them to the nParticleShape node using Create Ramp or Add Dynamic Attributes. See Set attributes on a per particle basis in the Dynamics guide.
For information about setting internal per-particle attribute ramps, see nParticle internal ramps and per-particle attributes.