The standard size for plywood sheets in North America is 4 by 8 feet. They are sold at home-improvement stores and are designed for indoor and outdoor applications. But for outdoor purposes, you need to use an exterior grade to ensure that it can hold up to water exposure.
Some sidings require just a basic layer of plywood. It's nailed to the frame of a house before the siding is installed. The plywood serves as the base for the siding. Not all sidings have the same requirements, however, and thicker, sturdier types of siding have lower deflection -- or flexibility -- requirements, which means that quarter-inch-thick plywood sheeting can be used.
Sturdier than quarter-inch sheets, half-inch plywood sheets are commonly used for home interiors, because they match the thickness of the most common type of drywall. In actuality, these sheets are five-eighths of an inch thick. They're also a common subfloor material in both indoor and outdoor settings. For walls, these are one of the sturdiest forms of plywood sheeting that you can choose.
If you live in an area where there are significant weather concerns, such as hurricanes or tornadoes, you may need to use a three-quarter-inch-thick material. This is the thickness used with sidings that need the firmest support available.