I agree, for most larger flowers (like 4" and up), you'd probably need a 20-25mm center. If you have the flowers already, put a coin in the center to get a rough idea of size. I don't have the diamters of the coins off the top of my head, but it would be super-easy to google. They correlate close enough to rhinestone size to get you in the ballpark, and to kind of get a visual
It really depends on the particular flower.
-Jennifer- Full-time working mommy to Connor (August '09) and Maia (June '11)
Please forgive my typos, I'm often on an iPad which is difficult to edit (if I even noticed it changed a word