The first thing you need to think about is the overall look of the room and if you have a theme, ie butterflies or a color, how that will fit into your centerpiece idea. The best way to get ideas is to hunt through wedding magazines and online to get pictures of the designs that you like.
Next you need to see how much space you will have at your tables, a 5ft round with 10 people on won't have much space for an elaborate centerpiece unless you have height rather than width. Whereas a 6ft round with 10 people on will have a fair amount of space to fill. If you have rectangular tables or 'longs' then you will have lots of space one way and not much the other, in other words you will need to have long centerpiece designs. Have a chat with your venue so that you can see how much room you have. Ask to see a room laid for another wedding, they will happily show it to you before the wedding party arrives.
The other consideration is the amount of other stuff on the table; menu, wine, water jugs, glasses and place settings. I have seen tables where the centerpiece is lost behind all the other extras. Before you go very high think how your guests will see each other at the table, the only way to test this is to make a mock up of the centerpiece and then sit at a table with it. Can you see your opposite guest? The top table?
Remember that the centerpiece doesn't have to be flowers or candles it can be anything you want; favor boxes stacked in the middle, pictures, rose petals, tall glass vases with peacock feathers, even baskets of chocolates.