Hi - if you have a request, please file an issue here: https://gitlab.com/gromacs
The functional form is described here:
http://manual.gromacs.org/current/reference-manual/functions/restraints.html?highlight=cylinder#flat-bottomed-position-restraints
Cylindrical restraints are under flat-bottom potentials, the restraint format is described here and contains the inputs for radius and force constant:
http://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html
The restraints have infinite length along the specified axis. If you really want to constrain a finite-length cylinder, you could add a few flat-bottom layer restraints along that axis.