When it comes to logo design, your main restriction has to do with your size and shape. Logos must be versatile and able to placed on business cards, websites and buildings. Because of this it's best to stick to a format that's either a perfect square or a rectangle (with width being the larger dimension). The reason for this is the format gives most versatility in placements. For example a logo that is taller then it is wide placed on the top of a website would stretch your header to be very large and consume a large portion of your screen making your website feel small.