Many florists insist that placing a mirror under the centerpiece doubles its size. I think this technique is not only outdated, but only works when viewing the centerpiece from standing height. Instead of using your money to rent or buy mirrors, consider placing white or ivory votives around a small centerpiece to expand the tableau. Not only will it visually enlarge your centerpiece, but it will lend a romantic light to your romantic evening.
I would recommend using three or more votives on each table. You can get away with using less on a round table - surrounding the centerpiece with three to five votives. On a rectangular table I recommend using six or more - splitting them in half and extending them out from each side of the centerpiece. This will allow you to fill the entire tablescape while using only one floral centerpiece. You can further emphasize the axis of the table with contrasting table runner on which the centerpiece and candles sit.
There are many places you can find inexpensive votives already in glass cups. Michaels and Bed Bath and Beyond carry votives by the dozen for about $8; however, I find their votives to be poorly made - often with wax running down the outside of the container. Jo-Ann Fabrics carries the nicest votives at a dozen for $10, and if you wait for a holiday they often go 30% off. Just make sure that you take a couple hours before the day of your wedding to remove the plastic that separates each votive from its glass cup!