Well, this is a handy link to determine the sizes you need:
http://www.wilton.com/wedding/cakeinfo/choosing/index.cfm
You need to determine the shape and whether or not they are saving the top tier for their first anniversary.
The size of the flowers you choose will largely be determined by the type of flowers you want, the look you want, the proportion to the size of the tiers you choose. For example if you choose roses for each tier, you might have them all the same size or you may have them graduated in size to be in proportion to the tier size with smaller ones on the top tier as borders or perhaps larger ones on top as a topper. A lot depends on the look you are going for, how close you want them in relationship to the actual real flowers, balance etc.
To get an idea of real size go to a florist and check out the real thing or purchase a few silk flowers and play with them against the size of the cake pan you will be using. Or cut a circle of paper to the sizes that gumpaste flowers come in, determine the type of arrangement you will be making and place on top of your cake pan to get an idea of the proportion and look.
If the only flowers on the cake will be the topper, you may want them a bit larger to stand out or you may want several smallers ones to have a more dainty appearance. a lot also depends on if the cake design is contemporary or traditional or Victorian.
Your best bet is to look at cake pictures and determine what appeals to you and go from there.
Hugs Squirrelly

That will give you more than enough, so that is good. The other thing with the flowers is that you may want to get a variety of sizes of flowers to give the topper depth?
Hugs Squirrelly