Is your room a 12x12? Living space? Foyer? Medallion going in the center? The 30" will provide a very nice accent to your floor.

Hi misphat8 - have you checked out http://www.houzz.com/pro/fingerblockfloors/custom-hardwood-supply? There's a nice selection of medallions in varying sizes. I agree you don't want the inlay to take over the floor. Owner Bill Adams is very knowlegable about inlays and sizes. Good luck! Whatever you do, be sure and post a photo later so we can see what you chose.