Q. How can I prune my very large Gardenia?
Well, that is a very large Gardenia, so you must be doing something right (summering it outdoors?).
Regarding pruning your gardenia plant, it is advised to do so in early-spring time to keep the shrubs low and bushy. Nip out growing points of any long new shoots on young plants, and cut out half or even two-thirds of the old wood of mature plants. Be careful, however, not to nip out flower buds. Stems can always be cut back later after the flowers have died. The cuts should be made immediately above points where growth-producing buds point outward rather then toward the center of the plant. Sounds okay, but you have to decide if you want the plant shorter, or as is, and trimmed a bit.
Hope this helps.