You must have at least 4MB of memory and 35MB of available hard disk space. If you want to install a reasonable amount of software, including the X Window system, and some development programs and libraries, you’ll need at least 300MB. For an essentially full installation, you’ll need around 800MB. To install everything available in Debian, you’ll probably need around 2GB. Actually, installing everything doesn’t make sense because some packages provide the same services.