For example, a recent show my art is entered into has the following parameters: the outside dimensions of the frame can not be larger than 42 square inches. The surface area size for miniature art is usually 25 inches.
Generally objects should be in 1/6th scale and portrayed in the spirit of miniature, with minute details that maintain a beautiful well-defined image in a magnifying glass. This 1/6th rule is a main difference between miniature art versus small works. For instance, if painting a red cardinal, for a small painting, the bird could be painted to fit a 5” x 7” frame.
But, to qualify as a miniature painting, the work must be painted 1/6th of the actual size of a real cardinal. The average length of a cardinal is 9”. Therefore, the miniature cardinal should measure be no longer than 1.5 inches in length and still have all the qualities and details in the finished painting as if it were painted actual size.
Find more information on the fascinating subject of miniature art, which dates back to the 7th century, here.