You may use any blank 'USB drive' with a minimum of 256mb storage capacity. You may also use an 'optical disk' (However, i recommend the use of a 'USB drive' for better reliability) Note: A recovery USB flash drive cannot be used to install Windows 8; it can be used to boot to the 'system recovery options' for Windows 8 recovery.
You may check the size of the 'System Image' and get a 'USB Media' that is large enough to hold it.
3. If you have a computer with a 'pre-installed copy of Windows' (OEM), you may want to check for a 'dedicated application' to create a 'back of system image'. The size of backup drive in this case will be determined by the size of 'System Image' created by the manufacturer. (These computers are generally pre-loaded with a proprietary 'Recovery System Image' and a 'Backup and Recovery Software')
Hope this answers your question. Do reply if you have any further questions; would be glad to assist.