Long table centerpieces are very easy to make on raquettes . . . but it does take a bit of time. The skill level is moderate, so it's easy to show friends how to help. If you want an alternative to using Raquettes (using bricks or cages instead) be sure to read through the pages on Choosing Your Foam.
You'll need to measure your tables and then choose the Oasis Raquette sizes you need.
Solid massed flowers like these garden roses can get rather pricey, as you'll need 45+ large headed flowers to create one 36" raquette. The raquettes can also be used as a base for silk flowers. The only difference is to dip the stems of the silk flowers in white glue before inserting into the foam.
Since the number of flowers needed can vary so much (using different sized flower varieties), I think it would be wise to buy two or three silk bushes of the flower variety you want to use and experiment on a brick of foam. Foam bricks are 9" long. Count how many flower heads it takes to cover it. It will give you a rough ideas of how many flower heads you need for an 18" raquette (2 bricks long), a 27" raquette (3 bricks long) and and 36" raquette (4 bricks long.) Keep in mind the size of flower heads. Nine peonies can cover one brick, yet it takes at least twice that to cover it with standard roses.
For a 6 foot banquet table and planning on a solid mass of flowers edging the table, you will need two 36" Raquettes on the front and an 18" raquette on each side as shown in the photo. It's advisable to wrap the raquettes with waterproof tape grid to help support these long of centerpieces. It will be quite heavy when saturated with water and loaded with fresh flowers.
You can measure in many different ways, since raquettes come in lengths of 18", 24" and 32". Be sure to read through the detailed tutorial on choosing the foam that works the best for you. The other tutorial which shows details on flowering and greening spray bars and and raquettes is also very helpful.