Round mirror centerpiece for a 72 inch round table

I've decorated for formal events where the venue lends out square mirrors for centerpieces. This can be a cost-effective option at a hardware store, versus hunting for circles. 12" x 12" is common and can reflect votives well.

The answer is probably more complicated than it seems.... Depending on how many people will be seated at your 6' tables and what your place settings/etc. will be, those tables may be very crowded. At the most, your centerpieces should probably fit within a 12" space. Eight people with place settings that include flatware and at least 2 glasses, and possibly a bread plate, and maybe a favor, will be pretty tight and there won't be a lot of room in the center of the table. Think about (or talk with your caterer about) what will be on the tables -- bread basket, butter, salad dressing, etc. All of that stuff will take up valuable space. Try to keep the footprint of your centerpieces small.