Formality doesn't rely on size. It relies on symmetry, calmness and repetition of patterns. The lawn is manicured, the hedges clipped and the flowers well mannered. There is definitely a plan apparent in a formal garden. Everything has a place and is in its place. While the garden may change with the seasons, the formal design is evergreen.
Urn or Birdbath
Place an urn or birdbath where it gets sun for most of the day. Surround the urn with a circle of red geraniums. Surround the red geraniums with white petunias and finally with an edging of blue lobelia. Plant the urn with a combination of the three flowers to repeat the colors and patterns of the small flower bed below. If you're using a birdbath as the focal point of this formal bed, keep it filled with clean fresh water. Scoop out the old water to the plants below.
Topiaries are shrubs that have been trained and clipped into shapes. Geometric shapes are popular because they're easier to maintain than animal shapes. Trim a row of bushes leading up to an entry way in graduating sizes of spheres. Place the largest spheres closest to the entry way. This graduation in sizes makes the entry way look longer and more impressive.
Standards are topiaries that are shaped like lollipops or a sphere on a stick. Roses make beautiful topiaries. They are roses grafted to a tall straight rose trunk from a different rose plant. Other flowers are trained to grow one trunk and are then pruned so the top fills out into a ball shape.
Most knot gardens take up quite a bit of space, but they don't have to. Use low-growing plants and herbs in a simple design. Outline a square with sweet alyssum. Connect the corners of the squares with arches made from growing lobelia. Connect the centers of each side of the square with arches of creeping thyme. Plant a specimen plant, perhaps a topiary, in the middle of the square.
Formal gardens often include artwork in the form of sculptures. Do the same thing in your small garden. Plant a circle of boxwood. Trim it to no more than 12 inches high and wide. Place the sculpture in the center of the boxwood. If the statue is short, say less than 24 inches high, put it on a group of cement blocks to elevate it. Fill in the circle of the boxwood with flowers that have a sculptural shape as well. Gerbera daisies work well because they come in a variety of bright colors and flower all summer long.