Personally I would go with 30" or larger depending on your preference. You have a beautiful dining room. What styles are you looking at?

Something like this:

https://www.houzz.com/products/palladium-crystal-fringe-5-tier-chandelier-prvw-vr~7939198

Guides I have read say the light diameter should be at 50 to 75% of table width, which would mean a 40 to 60 inch light? Yet going by the room dimensions of 12X14, it says I need a 26 inch diameter light. So what do you do in this case? In previous home I had a 28 inch diameter chandelier that seemed too small - it was a single tier. Do the multiple tiers make it seem larger?

The rule of thumb I have heard most for dining tables is about 1/2 the width of the table with some give or take for ceiling height and the visual weight of the chandelier. Room size can be a factor, as well as style and design trends -- and yes, the tiers add to the visual weight. I would mock up the chandelier you like and try scaling the picture of the chandelier to 32-26 inches -- about 40-45% of the table width, and hanging it a bit lower than the current one. The bottom of the chandelier is typically hung 30-36" from the top of the table.

How do you scale the chandelier and mock it up onto a picture? Thank you,

I'm sure there are better ways, but what I have done is printed out a picture of the room and then a picture of the fixture - in this case, sized to be slightly about half or slightly less than half the width of your table in the printed picture. I have also opened them in different tabs and pulled them up side by side. If you have photoshop or another program, you may be able to put them together as one photo.