Actual Size of 0.40 Carat Diamond
This page gives an illustrated example presenting the real size of a 0.40 carat diamond when set in a ring and put on a typical woman's hand. This image shows a size L 1/2 ring (Britain and other countries ), which is equivalent to a US size 6 .
It is good to be cautious when looking at a picture - a picture alone won't be entirely representative of all finger and hand types - you are best off getting an idea in store by trying on actual rings with your partner. The visual size of the diamond and ring will vary depending on whether you have thin or thick fingers, as well as by their length.
Don't make the mistake I made of examing a potential diamond engagement ring placed on my own, male finger - remember that it is going to go on a usually smaller and more petite female hand, which will make the diamond appear significantly larger.