You need to have enough drive space for your OS, Proxy server installation, and log files. In most cases, 2 Gbytes total is sufficient. Apart from that, you need to factor in the disk space required for Proxy server's disk cache.
Put the OS, swap or paging file, Proxy Server logs, and Disk cache each on separate hard drives. If your log files fill up the log drive, your OS does not suffer. Also, you will be able to tell whether, for example, the OS paging file is causing drive activity.
Your OS vendor can recommend how much swap or paging space to allocate. Based on testing, Proxy Server performs best with swap space equal to RAM, plus enough to map a size that corresponds to the frequently used portion of the disk cache.