All items have three common attributes. These are position, horizontal offset, and vertical offset. Guifications does not place items like most people are used to. You select a general position of where the item will be placed (Top Left, Center, Bottom Right, etc) and then place it exactly where you want it by using the horizontal and vertical offsets. As you can see in the screen shot, the offsets can also be used as percentages. All items use the notification's height and width to calculate the values of the percentages.
Icons have two unique attributes. These are type and size. The type is what type of icon to show, which includes protocol, status, and buddy icon. Size is the size to scale the icon too. You can choose from some previously defined values between 16x16 all the way to 144x144.