The Freespace Reference Bible states that a Fenris (and presumably a Leviathan) is 260 meters long, but doesn't give a crew complement. Since this is roughly one-tenth the size of a Hecate or Orion (both of which have crews of 10,000), you could say that a Fenris has a crew of about 100.
EDIT: The math alone suggests the actual number might be even smaller, as based on our known figures, ships seem to have an average of one crewman for every 4.5-5 meters long the ship is (which would give the Fenris a crew of 52-58). However, this might not take into account other personnel, like troops and pilots; we know for a fact that the original FS1 pilot was stationed aboard the Orff. Also, there's plenty of room to fudge the numbers; for comparison, the Constitution-class starship Enterprise, a heavy cruiser about the same length as the Fenris, had a crew of 400.