Electrical boxes comes in the floor mounted or wall mounted varieties. They are typically metal and relatively inexpensive. Of course, they come in various shapes and dimensions for all your electrical needs; there is no one "standard size".
The two most common shapes are the square, the rectangular, and the circular. The square and rectangular are more versatile and can be found in walls as well as floors. The circular ones are typically found in floors.
Floor and outlet electrical boxes may be labeled as for interior or exterior use or both. Some are specifically designed as "weatherproof". Exterior outlet boxes protects wiring from the elements such as rain and snow. You often find them protecting the wiring to outside lights in a patio or a parking lot.
You will often need an electrical box covers for your electrical box. They are needed to hide unsightly wirings. The are also used to protect the outlet when they are not in used. You don't want rain getting into your electrical socket.
You also need screws for your electrical boxes. Some screws can be self-tapping or low profile. Other accessories includes box support, mounting bracket, plaster ring, grounding pigtail, and wiring device retainer.