Since you are buying a mirrored tissue box cover, it's likely that it will feature straight lines rather than curved ones. Thus, your typical options are square, horizontal-laying rectangle and the tower type.
Square - This mirrored tissue box cover will have equal lengths on all sides. It's an excellent choice if you have limited space to work with.
Horizontal-Laying Rectangle - This one comes in a shape of a rectangle, but has a low profile. This is perhaps the most common type of mirrored tissue box cover. It can accommodate bigger sized tissue boxes, but may also consume more space.
Tower - This one is also shaped like a rectangle except it stands like a tower. This is an excellent choice if you plan to place the box on shorter tables. The extra height will make it easier for you to grab a tissue. On the downside, a tower type tends to fall down easily. You can counteract this issue with an adhesive on the bottom or any other means of fixing the box into place.