Three-dimensional giant molecular clusters, OPOSS16 and OPOSS24, have been designed and precisely synthesized. They have 16 or 24 octyl polyhedral oligomeric silsesquioxane (OPOSS) building blocks chemically linked by short, flexible chains. Despite the small difference in their molecular weight, 25 and 38 kg/mol respectively, they show different dynamics above the conventional glass transition temperature (T-g). OPOSS16 in the bulk quickly turns to viscous. In contrast, OPOSS24 shows a long-lasting elastic plateau even far above the T-g, corresponding to confinements on individual OPOSS. To achieve any confinements, clusters have to be immobile, although individual OPOSS possesses mobility. It directly confirms that in the bulk giant molecular clusters possess a cooperative soft-cluster glass in addition to conventional glass if they are larger than the critical diameter. This critical diameter is close to the estimated diameter of cooperative rearranging regions during glass transition. Giant molecular clusters present unique dynamics different from colloidal caging in solution or polymer entanglements.