Many devices say right on them how much storage space they come with. However, the actual space available to use may not be at 100%, depending on how much stuff is already on the device. The following link is specfic to flash drives, but the general concept will work for most devices that plug into a computer. Don't forget that you can try this one out using one of the flash drives from the supervisor's office!
How to Tell How Much Space Is on a Flash Drive