Scale, when used in your garden design is closely linked to size. Size on its own refers to a definite measurement, the dimensions of an object, it’s height, width and depth. Scale is the size of that object in relation to another object and to its surroundings generally. For instance, a large fountain would look out of scale in a small front garden of a single story house but is suitable in front of a historic palace in Paris.
The human scale is very important and you should consider it when designing spaces, structures and selecting plants. This is especially true for dividing spaces in larger gardens. If the scale of the space is too large a person standing in it will feel lost, disinterested and uncomfortable. The same can be said for a space that is too small as a person standing in the space can feel cramped and thus also uncomfortable. You must try and identify the mood that you wish to set in each space within your design and scale it accordingly; for instance small and intimate or large and open to the surrounding landscape.
Your garden is also a place for people, and their presence will also provide visual interest, colour and movement in the space. Whether people are using the space for active play, socialising or passive recreation, they should be part of the picture. Therefore when you are choosing planting, furniture and enclosures keep this in mind.
Tip: While you are designing, try to have your components, some of which will be plants, sculptures or enclosures, either lower or higher than the height of the average grown up human. Things in a garden space that are exactly the same height as the human frame tend to to be strangely less than visually pleasing in juxtaposition with ourselves.
Scale and balance are also related. You will often find that an entity such a planting scheme in the foreground is balanced against another entity such as the house in the background.
Experiment & see what works for you. The drawing by Patrick Ketch above shows four elements in relation to each other, a large tree, a human figure, a fence and a smaller shrub.