I don't know rule of thumb but my table is 60 inch so it's a lot smaller and I have a large Asian bowl, and a pair of double candlesticks and then about 12 to 16 inches left at either end. But that's because I spread them out. For just for looks you can go bigger than you would for a dinner party because no one is setting at the table!

Thank you! I am more torn on if I should do a round bowl or an oval/rectangle planter. I also want to do a blue and white Asian piece and trying to figure out what to purchase?!