Choose right color for the flowers:
Warm colored flowers like orange, yellow, red, brown if planted in the midst of the garden and cool colored flowers like blue, green, purple if planted along the boundaries of your garden, will give a spacious look to the garden.
Add a step:
This might seem surprising but a step can also help in giving your garden a broader look. It will create an illusion of larger space. Curves also make small spaces look spacious because there are no boundaries in sight.