Buoys used to provide safe navigation in harbours and channels used solely by small-craft are generally small. Some would say, minuscule. In good visibility and calm conditions they are hard to spot without binoculars. In choppy conditions, from the deck of a small boat, much harder. Their radar-return is minimal, so don't expect your radar to pick them up!
Typical, plastic buoys shown here are only a little larger than lobster trap buoys. They measure hardly 16 inches (40 cm) in diameter and float only 18 inches (45 cm) above the surface. They sink lower depending on the length of mooring chain and buildup of marine growth, as seen on the east cardinal mark, DC, shown below. As the cruising season progresses they often become white with seabird guano, making it difficult to distinguish their colours. The large lifting eye on the port-hand can may make it appear as a starboard-hand nun in poor light.
Although not indicated on CHS charts, Canadian Coast Guard does buoy many small harbours and anchorages. The chartlets on this website show these buoys where known to exist.