A standard size rectangular tissue box cover is 21.5 x 11.5 x 10.5 cm. However, mirrored tissue box covers come in many different sizes. To find the right size for you, begin by considering where you are planning to place the item. For example, if you want to put it on a table, visualize the right size in relation to the size of the tablespace and the other more permanent items on it. When you start shopping for a mirrored tissue box cover, always keep in mind the measurements.