grompp prints out an estimate of the amount of disk space that will be used, if it is above a certain threshold. Output frequency should be set to the minimum necessary to obtain reasonable statistics based on your needs.
Use the grompp estimate as a guideline, and make sure to have a lot of disk space available when doing MD. Trajectories are large. Consult http://www.gromacs.org/Documentation/How-tos/Reducing_Trajectory_Storage_Volume