These flower choices for vases are a guide and serve to give you a chance to feel the difference when determining how you want flowers to look.
1. First, determine how big you want to make your flower arrangement and how much space you want it to occupy on the table. Professional florists typically use their hands as a guide, similar to the way people talk about a fish they caught or the size of a piece of cake they ate. If you are making something tall for a bar, then think tall. If you are making a centerpiece for a table setting, then think low and lush. Usually, I like the flowers to be three to four times the width of the vase. The flowers should also be two to three times as wide as the vase is high.
2. If this more precise means of determining the size of your arrangement isn't working for you, then pick up your materials in one hand. Your flower arrangement will be about as large as all of the materials held in your hand, which means the opening of your vase needs to be big enough to hold all of the materials in your hand. This way of measuring isn't really mathematical, it's based on a florist's intuition and how much product you have.
3. As a rule of thumb, you should look at the opening of the vase to determine how many flowers will fit. Vases that have a small openings are good for taller flowers, and vases with wider openings are better for bushy flowers.
4. Visualize the faces of your flowers and how they will look best. Tulips look amazing upright, at attention, and with their stringing stems exposed. Hydrangeas (and garden roses, shown here) look good clustered together, often packed tightly, into a design.
Content reference from Flower School - Calvert Crary(Author) - Flower arranging