What size centrepieces do I need?

Hi ladies. Our venue has said that our tables will seat 10 people max, 6 minimum, but 8 is perfect. They are round tables. We are having a Wedding Breakfast, so being as it's a sit down meal, there will be crockery set out. We have unlimited house red and white wine with the meal, so I'm assuming there will be a few bottles of each on each table. We are also having a favour box and placecard for each adult on the table, and for each kid a play box, which are a little smaller than a happy meal box.

The big question is, what size table centrepieces do we need? I have been looking at fish bowls and mirror plates, and the different sizes are baffling me. Any advice would be gratefully received.

I really don't want the tables to look too cluttered, but I'm worried a centrepiece that's too small will look odd. x