You must have at least 32MB of memory and 110MB of hard disk space. For a minimal console-based system (all standard packages), 250MB is required. 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 400MB. For a more or less complete installation, you'll need around 800MB. To install everything available in Debian, you'll probably need around 6 GB. Actually, installing everything doesn't even make sense, since some packages conflict with others.