PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_LengthSpheroid(geometry a_geometry, spheroid a_spheroid);
func.ST_LengthSpheroid(geometry a_geometry, spheroid a_spheroid)
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation