Settings that make a diamond look larger
There are lots of little tricks and techniques that jewellers can employ to make a diamond look larger.
One such technique is to place a small mirror or reflective material beneath the diamond when setting the stone. The mirror helps the diamond look larger and deeper. (Do take care with mirror settings though as they are hard to repair). Wide or long shallow diamonds also look bigger than round deep diamonds even though they may have the same carat weight. You could have the stone cut into a pear, oval or marquise shape to make the surface look bigger. Ideally you could combine a wide shallow diamond set on top of a mirror, so that it looks wide and deep.
The way the diamond is set can also affect how big it actually looks. A bezel setting is where the diamond is raised up and encircled in metal, which makes it look larger.
Another trick to try is to surround the main diamond with smaller diamonds so that more of the surface of the ring is covered with stones.
Instead of smaller diamonds, you could go for a pave setting where there are literally hundreds of tiny diamonds set next to each other in the ring, making it look like a continuous diamond. Though the small diamonds cost less, it takes a lot of skill to place them into the ring in a pave setting, so it might end up more expensive.
The cut of the diamond also determines how sparkly it looks. The best cuts reflect all the light coming through the surface. Poorly cut stones lose some of the light. Emerald cuts have less sparkle than brilliant cuts (generally the more facets, the more sparkle). For a given carat weight, the only way to tell if a diamond is well cut is to inspect it carefully and ensure that it sparkles on all sides (it has been cut symetrically). The more colourless the diamond, the more sparkly it will seem (slight tinges of yellow can make the stone seem smaller).
The metal you choose can also enhance or diminish the diamond that it set in it. Platinum goes well with diamonds, especially if you are using a pave setting. Platinum is also very strong and will hold the stone securely. Yellow gold on the other hand provides a sharp colour contrast where the diamond ends and the metal starts and can make your diamond look smaller.
Finally make sure that the shape of the diamond suits the shape of your hand. If you have plump hands, a long diamond set horizontally can make the hands look shorter. Slim hands can make a diamond set in a slim band look bigger. There is no substitute for trying the rings on before you purchase them. It's impossible to judge looking at a picture in a catalogue or online.