Skateboards are measured by width. The narrower a board is, the lighter and more maneuverable it is for learning tricks. A wider board offers more foot space and stability. (Skateboards are generally all the same length.)
Kids: 7.75" width boards are recommended for young children because they are narrower and lighter. 7.75" - 8" boards are recommended for growing children with bigger feet.
Adults: Most adults will prefer 8" and up, but it becomes a personal preference the more you ride. 8" - 8.25" are the most common sizes. Great for beginners learning tricks as the board isn't too heavy and can accommodate an average foot size. For larger footed, taller riders, 8.25" - 8.5" is pretty standard.
Cruisers are sized similarly to skateboards but are generally wider (8.5"+) to accommodate larger wheels.
Skateboard trucks are sold as a pair. The size of trucks you choose is based on the size of board you'll be riding. The hanger width should not extend beyond the width of the board or else the wheels can be in the way while pushing. Here are some size charts for the trucks we carry: