Legend has it that the dimension of your credit cards, the index cards and the Greek Parthenon have one thing in common - the golden ratio. Not going into mathematics here but rather go direct to woodworking applications, the ratio that ancient Greeks deemed the ideal for the length to width proportion, or in the case of the Parthenon the width to height, is approximately 1.62. The 3X5 inch index card is close to that (5 divided by 3) and the credit card that is dimensioned at 3.375" X 2.125" is likewise close to that ratio.
It is not always possible to do everything in that ratio but keep it in mind when you begin to dimension your project if aesthetics is important. Common woodworking projects of tables, shelves and boxes, or picture frames, etc., are going to be rectangles or squares. With the latter, there is nothing to do but make all four pieces the same length. In a rectangle, where length and width are different, you might as well proportion them to the golden ratio, where possible, to make the dimension pleasing to the eyes.