The chair does not have to have the most knowledge about the topic to be considered by the group. It may be an advantage for the chair to have served on a committee previously, but not necessarily have served as a chair. Interest and willingness to serve are primary considerations. Persons with pertinent information should be given special consideration. The opportunity to provide the experience of learning by doing should not be overlooked.
Every member is a potential leader.
Often it's wise to have a cross section of the membership represented on the committee. Care should be taken not to overload key individuals. People who have demonstrated their unwillingness or inability to work together normally would not be assigned to the same committee. It is often good practice to ask chairs for suggestions because the responsibility rests with them. The purpose of the committee should be the primary consideration in determining how large the group should be.