This is a chart that will be extremely helpful for you to determine the ring size that fits your need best. You can either make use of a tape or a small piece of string to measure the area that will be occupied by the ring. When it becomes a complete circle, make a mark on such a string. This will be helpful for you to compare with the chart that is mentioned below. For extra width, you can add another size to accommodate. Alternatively, you can select an existing ring that fits your desired finger and measure the internal diameter of the ring on the ruler. You can then use the chart below to determine your ring size.
Ring Size Guide