For large arrangements you'll need flowers with long linear lines. Great choices are gladiolus, snapdragons, larkspur, and delphinium. Then fill in with larger headed flowers, such as open roses, lilies, carnations, chrysanthemums, hydrangeas and alstroemeria.
Don't feel limited by the examples of colors and flowers used for this floral. Experiment. Simply changing the flowers and colors can give the same basic design a totally different look.
Liatris is long with a firm stem and is great in large arrangements.
Before adding my flowers, I use the LeafShine to spray the greenery and give a nice glossy finish to the leaves that removes any white splotchy look.
I like my line flowers to be long and tall, reaching up and outward. Remember - what looks really big closeup doesn't look that overpowering from a distance! You need big arrangements to fill up big even places.
Straight up in the center, angle outwards and then angle downwards towards the bottom - just as you did with the greens.
Turn your arrangement around, filling in all sides unless this is to just be a one sides arrangement.
Place all of one flower variety at a time. This allows you to distribute it more evenly and mix up both your flowers and colors.
Don't be intimidated by large pieces - they can really be a lot of fun once you get into the design part!