In order for your floral arrangement to look balanced, the vase that you use should be approximately half the height of the flowers. If you plan to use a tall vase, your arrangement must be extremely tall so the flowers not only reach the top, but extend over the rim. As a rule, make sure your flowers are at least twice as long as the vase. Don't overlook the width either -- a circular arrangement will look uneven in a tall, narrow vase
Prior to arranging the flowers in the vase, strip all of the leaves that will be under water from the stems. In a tall vase, more of the stems may be covered by water, so you may need to strip more than usual. You may want to place the flowers in the vase to verify where the water will reach, so you don't unnecessarily strip leaves from the stems.
While the flowers in your arrangement should be at least twice as long as your vase, you should vary the height of the stems so some flowers are taller even. Having flowers of differing lengths gives the arrangement a fuller, more textured look. However, you may not want to use blooms of differing lengths if you are using a single type flower for the arrangement since it may create an uneven, messy look.
When arranging the flowers in your vase, place the tallest blooms in the center and the shortest along the rim. However, begin by placing the outer flowers and greenery first and work your way toward the center. Place the darkest flowers in the arrangement in the center and lighter blooms along the edge to create a more balanced look.
If your flowers are too short to fit your vase, a piece of florist foam may help remedy the problem. Soak the foam block in water for approximately half an hour to soften it, and place it in the bottom of your vase. As you arrange the flowers, push them into the foam. Because the block is fairly thick, it can raise the height of the flowers by an inch or so. It also has the added benefit of securing your arrangement in place. However, florist foam usually works only for flowers that have sturdy stems, such as lilies or chrysanthemums.